Semiconductor device and manufacturing method thereof

ABSTRACT

In a CMOS image sensor in which a plurality of pixels is arranged in a matrix, a transistor in which a channel formation region includes an oxide semiconductor is used for each of a charge accumulation control transistor and a reset transistor which are in a pixel portion. After a reset operation of the signal charge accumulation portion is performed in all the pixels arranged in the matrix, a charge accumulation operation by the photodiode is performed in all the pixels, and a read operation of a signal from the pixel is performed per row. Accordingly, an image can be taken without a distortion.

TECHNICAL FIELD

One embodiment of the present invention relates to a semiconductordevice in which pixels each provided with a photosensor are arranged ina matrix and to a driving method of the semiconductor device. Further,one embodiment of the present invention relates to an electronic deviceincluding the semiconductor device.

Note that the semiconductor device in this specification refers to alldevices that can function by utilizing semiconductor characteristics,and electro-optic devices, semiconductor circuits, and electronicdevices are all semiconductor devices.

BACKGROUND ART

As a semiconductor device in which pixels each provided with aphotosensor are arranged in a matrix, an image sensor is known. Theimage sensors are provided in many portable devices such as a digitalcamera or a cellular phone as imaging elements. In recent years, thedefinition of imaging has been increased, the portable devices have beendownsized, and power consumption has been reduced; therefore, a pixel inthe image sensor has been made smaller.

As an image sensor in general use, two kinds of sensors are known: acharge coupled device (CCD) sensor and a complementary metal oxidesemiconductor (CMOS) sensor. The CCD sensor is an image sensor in whichcharge is transmitted by a vertical CCD and a horizontal CCD. The CMOSsensor is an image sensor formed using a CMOS process. In the CMOSsensor, reading of charge can be controlled per pixel unit by switchingof a MOS transistor.

The CCD sensor has high sensitivity; however, when excessive light isincident on part of a photodiode, charge which is greater than or equalto the maximum permissible value flows into a vertically transfer CCD,and a longitudinal emission line, called a smear, is generated. Further,the CCD has problems such as high production cost with a dedicatedprocess and large power consumption due to many power sources.

In contrast, although the CMOS sensor has lower sensitivity than the CCDsensor, a general-purpose CMOS process can be utilized and circuits canbe integrated in one chip. Therefore, the CMOS sensor can achieve lowcost and low power consumption. Further, the CMOS sensor amplifies asignal in a pixel and outputs it; therefore, the influence of noise canbe reduced. In addition, because the method for transmitting charge bythe CMOS sensor is different from that by the CCD sensor, a smear is notgenerated.

However, for the conventional CMOS sensor, a rolling shutter system bywhich pixels arranged in a matrix are driven per row has been adopted.This rolling shutter system had a problem that an image was warped whenan image of an object which moves fast was taken. In contrast, for theCCD sensor, a global shutter system by which charge is accumulated atthe same time in all the pixels is adopted.

As a means in which the CMOS sensor with global shutter is used, PatentDocument 1 discloses a technique in which a mechanical shutter is usedtogether to control the current of a photodiode. In addition, PatentDocument 2 discloses a technique in which a path ejecting unwantedcharge which is generated in a photodiode after light exposure isterminated is provided to suppress leakage of accumulated charge.

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2006-191236-   [Patent Document 2] Japanese Published Patent Application No.    2004-111590

DISCLOSURE OF INVENTION

Reading of a CMOS sensor is performed by a sequential selection method.Because the time to read data from each pixel varies in the case of asensor with global shutter, a charge holding period gets longer as theorder of a pixel from which data is read is later.

When this charge holding period gets longer, charge flows out by leakagecurrent and the like of a transistor forming a pixel, so that originaldata is lost. In particular, there were problems that the leakage ofcharge becomes significant and charge cannot be held for a long periodof time when the off-state current of the transistor is high.

Therefore, in the techniques disclosed in the above-described patentdocuments, behavior of a photodiode is controlled by any method;however, the countermeasure against dark current of a photodiode has notbeen made. In addition, by addition of a mechanical shutter or of a newswitching element, there was a problem in high cost and a complicatedcontrol.

Thus, one embodiment of the present invention disclosed in thisspecification provides a structure of a pixel circuit which solves atleast one or more of the above-described problems, and a driving methodof a pixel.

One embodiment of the present invention relates to a semiconductordevice in which a photosensor with global shutter is included in each ofpixels arranged in a matrix, charge is accumulated in the pixels, andthe leakage of charge from an accumulated charge holding portion can besuppressed as much as possible in a period from the termination of anaccumulation period to the reading of the last row.

One embodiment of the present invention disclosed in this specificationis a semiconductor device including a plurality of pixels arranged in amatrix, and each of the plurality of pixels includes a photodiode, asignal charge accumulation portion, and a plurality of transistors. Achannel formation region in at least one or more of the plurality oftransistors includes an oxide semiconductor. After a reset operation ofthe signal charge accumulation portion is performed in all the pixelsarranged in the matrix at substantially the same time, a chargeaccumulation operation by the photodiode is performed in all the pixelsat substantially the same time, and a read operation of a signal fromthe pixels is performed per row.

The plurality of transistors is a charge accumulation control transistorof which one of a source and a drain is electrically connected to thephotodiode; a reset transistor of which one of a source and a drain iselectrically connected to the other of the source and the drain of thecharge accumulation control transistor; an amplifying transistor ofwhich a gate is electrically connected to the other of the source andthe drain of the charge accumulation control transistor and the one ofthe source and the drain of the reset transistor; and a selectiontransistor of which one of a source and a drain is electricallyconnected to the one of the source and the drain of the amplifyingtransistor.

The plurality of transistors may be a charge accumulation controltransistor of which one of a source and a drain is electricallyconnected to the photodiode; a reset transistor of which one of a sourceand a drain is electrically connected to the other of the source and thedrain of the charge accumulation control transistor; and an amplifyingtransistor of which a gate is electrically connected to the other of thesource and the drain of the charge accumulation control transistor andthe one of the source and the drain of the reset transistor.

Gates of the reset transistors in all the pixels in the above-describedtwo structures are electrically connected to each other, and thetransistors can be operated at the same time with one input signal.

The plurality of transistors may be a charge accumulation controltransistor of which one of a source and a drain is electricallyconnected to the photodiode; an amplifying transistor of which a gate iselectrically connected to the other of the source and the drain of thecharge accumulation control transistor; and a selection transistor ofwhich one of a source and a drain is electrically connected to one of asource and a drain of the amplifying transistor.

Gates of the charge accumulation control transistors in all the pixelsin the above-described three structures are electrically connected toeach other, and the transistors can be operated at the same time withone input signal.

The plurality of transistors may be an amplifying transistor of which agate is electrically connected to the photodiode; and a selectiontransistor of which one of a source and a drain is electricallyconnected to one of a source and a drain of the amplifying transistor.

Another embodiment of the present invention disclosed in thisspecification is a semiconductor device including a plurality of pixelsarranged in a matrix, and each of the plurality of pixels includes aphotodiode, a signal charge accumulation portion, a transistor, and acapacitor. A channel formation region in the transistor includes anoxide semiconductor. After a reset operation of the signal chargeaccumulation portion is performed in all the pixels arranged in thematrix at substantially the same time, a charge accumulation operationby the photodiode is performed in all the pixels at substantially thesame time, and a read operation of a signal from the pixels is performedper row.

The transistor is an amplifying transistor of which a gate iselectrically connected to the photodiode and one electrode of thecapacitor.

Here, in the transistor whose channel formation region includes an oxidesemiconductor, a highly-purified oxide semiconductor layer with a veryfew carriers is employed. Specifically, in the transistor including theoxide semiconductor layer, off-state current density per micrometer in achannel width at room temperature can be less than or equal to 10 aA(1×10⁻¹⁷ A/μm), further less than or equal to 1 aA (1×10⁻¹⁸ A/μm), orstill further less than or equal to 10 zA (1×10⁻²⁰ A/μm). In particular,it is preferable to use a transistor including an oxide semiconductorfor the charge accumulation control transistor and/or the resettransistor in order to prevent leakage of charge from the signal chargeaccumulation portion.

Another embodiment of the present invention disclosed in thisspecification is a driving method of a semiconductor device including aplurality of pixels arranged in a matrix, each of the plurality ofpixels including a photodiode, a charge accumulation control transistorof which one of a source and a drain is electricity connected to thephotodiode, a reset transistor of which one of a source and a drain iselectricity connected to the other of the source and the drain of thecharge accumulation control transistor, an amplifying transistor ofwhich a gate is electrically connected to the other of the source andthe drain of the charge accumulation control transistor and the one ofthe source and the drain of the reset transistor, and a selectiontransistor of which one of a source and a drain is electricity connectedto one of a source and a drain of the amplifying transistor. The drivingmethod includes the steps of: turning on the charge accumulation controltransistor in each of the pixels; turning on the reset transistor ineach of the pixels, and setting a potential of a signal chargeaccumulation portion in each of the pixels to a reset potential; turningoff the reset transistor in each of the pixels, and changing thepotential of the signal charge accumulation portion in each of thepixels; turning off the charge accumulation control transistor in eachof the pixels, and holding the potential of the signal chargeaccumulation portion in each of the pixels; and turning on the selectiontransistor per row sequentially, and outputting a signal correspondingto the potential of the signal charge accumulation portion in each ofthe pixels from the amplifying transistor in each of the pixels.

Another embodiment of the present invention disclosed in thisspecification is a driving method of a semiconductor device including aplurality of pixels arranged in a matrix, each of the plurality ofpixels including a photodiode, a charge accumulation control transistorof which one of a source and a drain is electricity connected to thephotodiode, a reset transistor of which one of a source and a drain iselectricity connected to the other of the source and the drain of thecharge accumulation control transistor, an amplifying transistor ofwhich a gate is electrically connected to the other of the source andthe drain of the charge accumulation control transistor and the one ofthe source and the drain of the reset transistor, and a selectiontransistor of which one of a source and a drain is electricity connectedto one of a source and a drain of the amplifying transistor. The drivingmethod includes the steps of: turning on the charge accumulation controltransistor in each of the pixels; turning on the reset transistor ineach of the pixels, and setting a potential of a signal chargeaccumulation portion in each of the pixels to a reset potential; turningoff the charge accumulation control transistor in each of the pixels,and changing a potential of a cathode of the photodiode in each of thepixels; turning off the reset transistor in each of the pixels, andholding the potential of the signal charge accumulation portion in eachof the pixels; turning on the charge accumulation control transistor ineach of the pixels, and changing the potential of the signal chargeaccumulation portion in each of the pixels; turning off the chargeaccumulation control transistor in each of the pixels, and holding thepotential of the signal charge accumulation portion; and turning on theselection transistor per row sequentially, and outputting a signalcorresponding to the potential of the signal charge accumulation portionin each of the pixels from the amplifying transistor in each of thepixels.

According to one embodiment of the present invention, a CMOS imagesensor in which the leakage of charge from the accumulated chargeholding portion can be suppressed as much as possible in a period fromthe termination of the accumulation period to the reading of the lastrow and in which an image without a distortion can be taken can beprovided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIGS. 2A and 2B are timing charts each illustrating the operation of apixel circuit in an image sensor.

FIG. 3 is a timing chart illustrating the operation of a pixel circuitin an image sensor.

FIG. 4 is a timing chart illustrating the operation of a pixel circuitin an image sensor.

FIGS. 5A to 5C are diagrams illustrating examples of images taken bysensors with rolling shutter and global shutter.

FIG. 6 is a diagram for illustrating scientific calculation.

FIGS. 7A to 7D are diagrams illustrating results of scientificcalculation.

FIG. 8 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 9 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIGS. 10A and 10B are timing charts each illustrating the operation of apixel circuit in an image sensor.

FIG. 11 is a top view illustrating a layout of a pixel circuit in animage sensor.

FIG. 12 is a cross-sectional view illustrating a layout of a pixelcircuit in an image sensor.

FIG. 13 is a top view illustrating a layout of a pixel circuit in animage sensor.

FIG. 14 is a cross-sectional view illustrating a layout of a pixelcircuit in an image sensor.

FIG. 15 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 16 is a top view illustrating a layout of a pixel circuit in animage sensor.

FIG. 17 is a cross-sectional view illustrating a layout of a pixelcircuit in an image sensor.

FIG. 18 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 19 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 20 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIGS. 21A and 21B are timing charts each illustrating the operation of apixel circuit in an image sensor.

FIG. 22 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIGS. 23A and 23B are timing charts each illustrating the operation of apixel circuit in an image sensor.

FIG. 24 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIGS. 25A and 25B are timing charts each illustrating the operation of apixel circuit in an image sensor.

FIG. 26 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 27 is a timing chart illustrating the operation of a pixel circuitin an image sensor.

FIG. 28 is a diagram illustrating a circuit configuration of a pixel inan image sensor.

FIG. 29 is a timing chart illustrating the operation of a pixel circuitin an image sensor.

FIGS. 30A to 30D are cross-sectional views each illustrating a structureof a transistor.

FIGS. 31A to 31E are cross-sectional views illustrating a manufacturingprocess of a transistor.

FIGS. 32A and 32B are diagrams each illustrating a circuit configurationof a pixel in an image sensor.

FIG. 33 is a timing chart illustrating an input signal of an imagesensor.

FIGS. 34A and 34B are diagrams each illustrating an output signal of animage sensor.

FIGS. 35A and 35B are diagrams each illustrating an output signal of animage sensor.

FIGS. 36A to 36D are diagrams each illustrating a specific example of anelectronic device.

FIG. 37 is a diagram expressing a relation between the number of grayscales of a taken image and charge.

FIG. 38 is a diagram expressing a relation between off-state current andframe frequency of a transistor which is necessary for holding charge.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments and examples of the present invention will bedescribed in detail with reference to the accompanying drawings.However, the present invention is not limited to the description below,and it is easily understood by those skilled in the art that modes anddetails disclosed herein can be modified in various ways withoutdeparting from the spirit and scope of the present invention. Therefore,the present invention is not construed as being limited to descriptionof the embodiments and examples. In the drawings for describing theembodiments and examples, the same portions or portions having a similarfunction are denoted by the same reference numerals, and description ofsuch portions is not repeated.

Note that this specification, a CMOS sensor is a name used to bedistinguished from a CCD sensor and refers to all image sensors formedusing a general process of a field effect transistor. Therefore, oneembodiment of the present invention is not limited to the case where aCMOS circuit is used in a pixel portion or a peripheral circuit portion.

Embodiment 1

In this embodiment, a semiconductor device which is one embodiment ofthe present invention will be described with reference to drawings. FIG.1 is an example of a circuit configuration of a pixel in an imagesensor.

The pixel in the image sensor includes a photodiode 101 (PD), anamplifying transistor 102 (AMP), a charge accumulation controltransistor 103 (T), a reset transistor 104 (R), and a selectiontransistor 105 (S).

Next, functions and arrangement of elements and wirings are described.

The photodiode 101 generates current in accordance with the amount oflight incident on the pixel. The amplifying transistor 102 outputs asignal which corresponds to a potential of a signal charge accumulationportion 112 (FD). The charge accumulation control transistor 103controls charge accumulation in the signal charge accumulation portion112 performed by the photodiode 101. The reset transistor 104 controlsthe initialization of the potential of the signal charge accumulationportion 112. The selection transistor 105 controls the selection of thepixel in reading. The signal charge accumulation portion 112 is a chargeholding node and holds charge which varies depending on the amount oflight received by the photodiode 101.

A charge accumulation control signal line 113 (TX) is a signal linewhich controls the charge accumulation control transistor 103. A resetsignal line 114 (RS) is a signal line which controls the resettransistor 104. A selection signal line 115 (SE) is a signal line whichcontrols the selection transistor 105. An output signal line 120 (OUT)is a signal line serving as an output destination of a signal generatedby the amplifying transistor 102. A power supply line 130 (VDD) is asignal line which supplies power supply voltage. A ground potential line131 (GND) is a signal line which sets a reference potential.

Note that transistors and wirings are named for convenience. Any ofnames is acceptable as long as the transistors have the functionsdescribed above and the wirings have the functions as described above.

A gate of the charge accumulation control transistor 103 is connected tothe charge accumulation control signal line 113, one of a source and adrain of the charge accumulation control transistors 103 is connected toa cathode of the photodiode 101, and the other of the source and thedrain of the charge accumulation control transistors 103 is connected tothe signal charge accumulation portion 112. In addition, an anode of thephotodiode 101 is connected to the ground potential line 131. Here, acharge holding capacitor may be connected between the signal chargeaccumulation portion 112 and the ground potential line 131.

Note that although a substantial signal charge accumulation portion isthe capacitance of a depletion layer in the vicinity of a source regionor drain region of a transistor, the gate capacitance of an amplifyingtransistor, or the like, the signal charge accumulation portion isconveniently described as part of a circuit diagram in thisspecification. Therefore, description of arrangement should follow thecircuit diagram.

A gate of the amplifying transistor 102 is connected to the signalcharge accumulation portion 112, one of a source and a drain of theamplifying transistor 102 is connected to the power supply line 130, andthe other of the source and the drain of the amplifying transistor 102is connected to one of a source and a drain of the selection transistor105.

A gate of the reset transistor 104 is connected to the reset signal line114, one of a source and a drain of the reset transistor 104 isconnected to the power supply line 130, and the other of the source andthe drain of the reset transistor 104 is connected to the signal chargeaccumulation portion 112.

A gate of the selection transistor 105 is connected to the selectionsignal line 115, and the other of the source and the drain of theselection transistor 105 is connected to the output signal line 120.

Next, a structure of each element illustrated in FIG. 1 is described.

The photodiode 101 can be formed using a silicon semiconductor with a pnjunction or a pin junction. Here, a pin photodiode in which an i-typesemiconductor layer is formed using amorphous silicon is used. Ifamorphous silicon is used, the amorphous silicon has optical absorptionproperties in a visible light wavelength region; therefore, a visiblelight sensor in which an infrared ray cut filter does not have to beprovided can be formed at low cost. In contrast, because crystallinesilicon also has optical absorption properties in an infrared wavelengthregion, when an i-type semiconductor layer of a pin photodiode is formedusing crystalline silicon and the pin photodiode is combined with aninfrared ray transmission filter, only infrared rays can be detected.

Although the charge accumulation control transistor 103, the resettransistor 104, the amplifying transistor 102, and the selectiontransistor 105 can also be formed using silicon semiconductor, these arepreferably formed using an oxide semiconductor. A transistor includingan oxide semiconductor has very low off-state current.

In particular, if the charge accumulation control transistor 103 and thereset transistor 104 which are connected to the signal chargeaccumulation portion 112 have large leakage current, the time whencharge can be held in the signal charge accumulation portion 112 is notsufficient; therefore, at least the transistors are preferably formedusing an oxide semiconductor. When a transistor including an oxidesemiconductor is used for the transistors, unwanted leakage of chargethrough the photodiode can be prevented.

For the oxide semiconductor, a thin film represented by the chemicalformula, InMO₃(ZnO)_(m) (m>0) can be used. Here, M represents one ormore metal elements selected from Ga, Al, Mn, and Co. For example, M canbe Ga, Ga and Al, Ga and Mn, Ga and Co, or the like. Because atransistor is formed using an oxide semiconductor, the off-state currentcan be drastically reduced.

Next, the operation of the pixel circuit of FIG. 1 is described withreference to timing charts illustrated in FIGS. 2A and 2B.

For simple description in FIGS. 2A and 2B, a potential 213 of the chargeaccumulation control signal line 113, a potential 214 of the resetsignal line 114, and a potential 215 of the selection signal line 115are provided as signals which vary between two levels. Note that becauseeach potential is an analog signal, the potential can, in practice, havevarious levels in accordance with situations without limitation on twolevels.

First, an operation mode according to FIG. 2A is described.

When the potential 213 of the charge accumulation control signal line113 is set to a high level at time 230, and then the potential 214 ofthe reset signal line 114 is set to a high level at time 231, apotential 212 of the signal charge accumulation portion 112 isinitialized to a potential of the power supply line 130 to be a resetpotential. The above is a start of a reset operation.

The potential 214 of the reset signal line 114 is set to a low level attime 232, and the reset operation is terminated. At this time, thepotential 212 of the signal charge accumulation portion 112 is held, anda reverse bias voltage is applied to the photodiode 101. This stagebecomes a start of an accumulation operation. Then, reverse currentcorresponding to the amount of light flows to the photodiode 101, andthe potential 212 of the signal charge accumulation portion 112 varies.

When the potential 213 of the charge accumulation control signal line113 is set to a low level at time 233, a transfer of charge from thesignal charge accumulation portion 112 to the photodiode 101 stops, andthe potential 212 of the signal charge accumulation portion 112 isdetermined. At this stage, the accumulation operation is terminated.

When the potential 215 of the selection signal line 115 is set to a highlevel at time 234, charge is supplied from the power supply line 130 tothe output signal line 120 in accordance with the potential 212 of thesignal charge accumulation portion 112, and a read operation starts.

When the potential 215 of the selection signal line 115 is set to a lowlevel at time 235, charge supplied from the power supply line 130 to theoutput signal line 120 is stopped, and a potential 220 of the outputsignal line is determined. At this stage, the read operation isterminated. After that, an operation returns to the operation at thetime 230, and the same operations are repeated, whereby an image can betaken.

Next, an operation mode according to FIG. 2B is described.

When the potential 213 of the charge accumulation control signal line113 is set to a high level at the time 230 and the potential 214 of thereset signal line 114 is set to a high level at the time 231, thepotential 212 of the signal charge accumulation portion 112 and apotential of the cathode of the photodiode 101 are initialized to thepotential of the power supply line 130 to be the reset potential. Theabove is a start of the reset operation.

When the potential 213 of the charge accumulation control signal line113 is set to a low level at time 236 and then the potential 214 of thereset signal line 114 is set to a low level at time 237, the resetoperation is terminated; accordingly, reverse current corresponding tothe amount of light flows to the photodiode to which the reverse biasvoltage is applied, whereby the potential of the cathode of thephotodiode 101 varies.

When the potential 213 of the charge accumulation control signal line113 is set to a high level again at the time 232, current flows by adifference in potential between the signal charge accumulation portion112 and the cathode of the photodiode 101, and the potential 212 of thesignal charge accumulation portion 112 varies.

The steps after that are the same as those of the operation modeaccording to FIG. 2A.

As a system of the accumulation operation and the read operation in allthe pixels, the following two systems are known: a rolling shuttersystem, and a global shutter system. Differences of these systems arebriefly described using a potential of the charge accumulation controlsignal line and a potential of the selection signal line.

FIG. 3 is a timing chart of the case where the rolling shutter system isused. First, a potential 3001 of the first charge accumulation controlsignal line is set to a high level, and charge corresponding to theamount of light is accumulated in the signal charge accumulation portionin the pixel of the first row in an accumulation period 301. Next, thepotential 3001 of the first charge accumulation control signal line isset to a low level, and a potential 3501 of the first the selectionsignal line is set to a high level after a charge holding period 302.After voltage corresponding to an accumulation potential is read in aperiod 303, the potential 3501 of the first the selection signal line isset to a low level.

In the period 303, a potential 3002 of the second charge accumulationcontrol signal line is set to a high level, and charge corresponding tothe amount of light is accumulated in the signal charge accumulationportion in the pixel of the second row. Next, the potential 3002 of thesecond charge accumulation control signal line is set to a low level,and a potential 3502 of the second the selection signal line is set to ahigh level after a charge holding period 304. After voltagecorresponding to the accumulation potential is read in a period 305, thepotential 3502 of the second the selection signal line is set to a lowlevel.

Similarly, when the last row is, for example, the 480th row, potentialsfrom a potential 3003 of the third charge accumulation control signalline to a potential 3480 of the 480th charge accumulation control signalline and potentials from a potential 3503 of the third selection signalline to a potential 3980 of the 480th selection signal line arecontrolled sequentially, whereby the read operation in all the pixels isperformed. In this way, reading of one frame is completed.

In the rolling shutter system, charge accumulation to the signal chargeaccumulation portion in the pixel is performed per row; therefore, thetiming of charge accumulation is different from each row. In otherwords, the rolling shutter system is a system in which the chargeaccumulation operation is not performed in all the pixels at the sametime and a time difference of the accumulation operation occurs per row.Note that the charge holding period from the accumulation operation tothe read operation is the same in all the rows.

Next, the global shutter system is described using a timing chart ofFIG. 4. Similarly to the above-described example, when the last row isthe 480th row, potentials from a potential 4001 of the first chargeaccumulation control signal line of the first row to a potential of the480th charge accumulation control signal line of the 480th row are setto high levels at the same time, whereby the charge accumulationoperation is performed in all the pixels in a period 401 at the sametime. In a period 403 after a charge holding period 402, a potential4501 of the first the selection signal line is set to a high level, andthe pixel of the first row is selected, whereby voltage corresponding tothe accumulation potential is output.

Next, the potential 4501 of the selection signal line is set to a lowlevel. In a period 405 after a charge holding period 404, a potential4502 of the second the selection signal line is set to a high level, andthe pixel of the second row is selected, whereby voltage correspondingto the accumulation potential is output.

After that, reading of each row is performed sequentially. In the lastrow, a potential 4980 of the 480th selection signal line is set to ahigh level after a charge holding period 406, and the pixel of the 480throw is selected, whereby voltage corresponding to the accumulationpotential is output. In this way, reading of one frame is completed.

In the global shutter system, the timing of the charge accumulation tothe signal charge accumulation portion is the same in all the pixels.Note that the period of time from the charge accumulation operation tothe read operation is different from each row, and the charge holdingperiod 406 up to the reading of the last row is the longest.

As described above, the global shutter system is advantageous in that animage can be taken without a distortion with respect to an object withmovement because there is no time difference of the charge accumulationin all the pixels. However, a charge holding period is increased usingthe global shutter system; therefore, there is a problem that an imagetaken by a sensor with global shutter is easily affected by leakage dueto the off-state current or the like of the charge accumulation controltransistor or the reset transistor, as compared to an image taken by asensor with rolling shutter.

Next, examples of images taken by sensors with rolling shutter andglobal shutter are described with reference to FIGS. 5A to 5C. Here, asan example of the case where an object moves fast, the case where animage of a moving car as illustrated in FIG. 5A is taken is considered.

In the case where the rolling shutter system is used, the timing of thecharge accumulation of the pixel is different from each row; therefore,imaging of the upper part of an image and imaging of the lower part ofthe image cannot be performed at the same time, and the image isgenerated as a distorted object as illustrated in FIG. 5B. In therolling shutter system, a distortion of a taken image increases inparticular when an object which moves fast is perceived; therefore, itis difficult to take an image of the actual shape of the object.

In contrast, in the case where the global shutter system is used, thetiming of the charge accumulation of the pixel is the same in all thepixels. Therefore, because the whole frame can be taken instantaneously,an image without a distortion as illustrated in FIG. 5C can be taken.The global shutter system is an excellent system for taking an image ofan object which moves fast.

As described above, it is found that not the rolling shutter system butthe global shutter system is suitable for taking an image of an objectwhich moves fast. Note that the conventional transistor used for a CMOSimage sensor has large off-state current; therefore, a normal imagecannot be taken by the CMOS image sensor with just global shutter.

Thus, in one embodiment of the present invention, a transistor includingan oxide semiconductor, of which off-state current is extremely low, isused for a CMOS image sensor with global shutter, whereby a normal imagecan be taken.

Next, scientific calculation results on an image are described. Anobject used for the scientific calculation is an image with threeblades, which is to serve as a rotor illustrated in FIG. 6. These threeblades can rotate using a connection point of the blades as a centralaxis. This scientific calculation aims at taking an image for one framewhen an image of three rotating blades is taken.

The software used for the scientific calculation is image processingsoftware written in C language, which is used for calculating the timingof a charge accumulation operation and of a read operation in each pixelof an image sensor and the amount of leakage from a signal chargeaccumulation portion per row to create a picture.

FIGS. 7A to 7D illustrate the scientific calculation results. Note thatthe scientific calculation was performed under the following fourconditions.

The first condition is to drive the VGA-size image sensor with rollingshutter, which has a pixel circuit illustrated in FIG. 8. Although thepixel circuit configuration of FIG. 8 is basically the same as that ofthe pixel circuit of FIG. 1, a charge accumulation control transistor1803, a reset transistor 1804, an amplifying transistor 1802, and aselection transistor 1805 are transistors including a siliconsemiconductor. Note that the operation of the pixel circuit includingthe following conditions is similar to that described with reference toFIG. 1 and FIGS. 2A and 2B.

The second condition is to drive the VGA-size image sensor with globalshutter, which has the pixel circuit of FIG. 8. The structure of thecircuit is the same as that of the first condition except for a shuttersystem.

The third condition is to drive the VGA-size image sensor with rollingshutter, which has a pixel circuit of FIG. 9. Although a pixel circuitconfiguration of FIG. 9 is basically the same as that of the pixelcircuit of FIG. 1, a charge accumulation control transistor 1903 and areset transistor 1904 are transistors including an oxide semiconductor,whereas an amplifying transistor 1902 and a selection transistor 1905are transistors including a silicon semiconductor.

The fourth condition is to drive the VGA-size image sensor with globalshutter, which has the pixel circuit of FIG. 9. The structure of acircuit is the same as that of the third condition except for a shuttersystem.

Note that each transistor including a silicon semiconductor in the pixelcircuits of FIG. 8 and FIG. 9 had a channel length L of 3 [μm], achannel width W of 5 [μm], and a thickness d of a gate insulating filmof 20 [nm]. In addition, each transistor including an oxidesemiconductor had a channel length L of 3 [μm], a channel width W of 5[μm], and a thickness d of a gate insulating film of 200 [nm].

Further, an imaging frequency was set to 60 [Hz], and the electricalcharacteristics of the transistor including a silicon semiconductorsatisfied Icut=10 [pA], and the electrical characteristics of thetransistor including an oxide semiconductor satisfied Icut=0.1 [aA]. Theterm Icut in this embodiment means the amount of current flowing betweena source and a drain when gate voltage is set to 0 V and drain voltageis set to 5 V.

The condition of rotational movement of the three blades shown in FIG. 6was set to 640 [rpm] in a clockwise direction. Note that when the numberof rotations is 640 [rpm], the three blades rotate by approximately 60degrees during one frame (1/60 [s]) at the time of the accumulationoperation of the rolling shutter.

In the case of the first condition (the transistors were only siliconsemiconductor transistors and the rolling shutter system was used), thetiming to accumulate charge in the signal charge accumulation portion ofthe pixel is different from each row; therefore, a distortion occurs inan image as illustrated in FIG. 7A.

In the case of the second condition (the transistors were only siliconsemiconductor transistors and the global shutter system was used), achange of gray scale is seen as illustrated in FIG. 7B, which is causedby charge leakage due to the off-state current of the chargeaccumulation control transistor 1803 and the reset transistor 1804. Thecharge holding period gets longer as a read operation is closer to theread operation of the last row on the lower side in the image sensorwith global shutter; therefore, the change becomes remarkable.

In the case of the third condition (the charge accumulation controltransistor and the reset transistor were oxide semiconductortransistors, and the rolling shutter system was used), an image isdistorted as illustrated in FIG. 7C, which is similar to the case of thefirst condition.

In the case of the fourth condition (the charge accumulation controltransistor and the reset transistor were oxide semiconductortransistors, and the global shutter system was driven), there is littlecharge leakage due to the off-state current of the transistor and thegray scale is properly displayed as illustrated in FIG. 7D as in FIG. 6.

It is found from the results illustrated in FIGS. 7A to 7D that therolling shutter causes an image distortion in either pixel circuit ofFIG. 8 or FIG. 9 and there is no strong correlation between the imagedistortion and the off-state current. In other words, in order todecrease the image distortion, it is effective to drive an image sensorwith global shutter by which the timing to accumulate charge in thesignal charge accumulation portion of the pixel is the same in all thepixels.

In contrast, when a circuit is formed using a conventional transistorincluding a silicon semiconductor, it is found that the global shuttersystem has a problem that a gray scale varies due to charge leakage dueto the off-state current of the charge accumulation control transistorand the reset transistor.

In one embodiment of the present invention, a transistor including anoxide semiconductor showing characteristics of extremely low off-statecurrent is used for each of the charge accumulation control transistorand the reset transistor in order to solve this problem. Therefore, theglobal shutter system can be adopted for a CMOS image sensor, and evenan image of an object with movement can be taken without a distortion.

Next, an example of a peripheral circuit in the case where an imagesensor with global shutter is used in this embodiment is described.

In an image sensor with rolling shutter, in order to accumulate and reada signal potential per row, a high-performance sequential circuit suchas a shift register was used for each of a gate driver circuit for acharge accumulation control signal line and a driver circuit for a resetsignal line, for example.

In one embodiment of the present invention, the charge accumulationcontrol transistors in all the pixels operate at the same time becausethe global shutter system is used. Therefore, a sequential circuit isnot needed for the operation of the transistors. In addition, the samecan be applied to the reset transistors.

That is, the number of driver circuits for a charge accumulation controlsignal line and driver circuits for a reset signal line which are formedwith sequential circuits such as shift registers can be reduced. Astructure may be used in which gates of the charge accumulation controltransistors in all the pixels are electrically connected to each other,gates of the reset transistors in all the pixels are electricallyconnected to each other, and the charge accumulation control transistorsin all the pixels or the reset transistors in all the pixels areoperated at the same time with one signal.

With this structure, power consumption of the semiconductor device canbe reduced, and further, an area required for the driver circuits can begreatly reduced. In addition, the area of the wiring can be reduced;therefore, flexibility in layout of the charge accumulation controlsignal line and the reset signal line can be improved.

Next, a driving method of a semiconductor device with theabove-mentioned structure is described with reference to FIGS. 10A and10B. Note that a VGA-size semiconductor device in which the number ofrows in a pixel matrix is 480 is used as an example here.

For simple description in FIGS. 10A and 10B, a potential 3613 of thecharge accumulation control signal line 113, a potential 3614 of thereset signal line 114, and potentials of a potential 36001 of the firstselection signal line 115 to a potential 36480 of the 480th selectionsignal line are provided as signals which vary between two levels. Notethat because each potential is an analog signal, the potential can, inpractice, have various levels in accordance with situations withoutlimitation on two levels.

First, an operation mode according to FIG. 10A is described.

The potential 3613 of the charge accumulation control signal line 113 isset to a high level at time 3631. Next, when the potential 3614 of thereset signal line 114 is set to a high level at time 3632, the resetoperation of the pixels from the first row to the 480th row iscompleted.

When the potential 3614 of the reset signal line 114 is set to a lowlevel at time 3633, the charge accumulation operation in the signalcharge accumulation portion 112 starts in all the pixels from the firstrow to the 480th row.

When the potential 3613 of the charge accumulation control signal line113 is set to a low level at time 3634, the accumulation operation isterminated in all the pixels from the first row to the 480th row.

When the potential 36001 of the first the selection signal line 115 isset to a high level at time 3635, the read operation of chargeaccumulated in the signal charge accumulation portion 112 in the pixelof the first row starts.

When the potential 36001 of the first the selection signal line is setto a low level at time 3636, the read operation with respect to thepixel of the first row is completed.

When the potential 36002 of the second the selection signal line 115 isset to a high level at time 3637, the read operation of chargeaccumulated in the signal charge accumulation portion 112 in the pixelof the second row starts.

When the potential 36002 of the second the selection signal line 115 isset to a low level at time 3638, the read operation of the pixel of thesecond row is completed.

Similarly, signals are sequentially transmitted up to the potential36480 of the 480th selection signal line 115, and the read operationwith respect to all the pixels is performed, whereby a first frame imageis obtained. After that, the operation returns to the operation at thetime 3631, and the same operations are repeated, whereby a second frameimage and subsequent frame images can be obtained.

Next, an operation mode according to FIG. 10B is described.

The potential 3613 of the charge accumulation control signal line 113 isset to a high level at the time 3631. Next, when the potential 3614 ofthe reset signal line 114 is set to a high level at the time 3632, thepixels from the first row to the 480th row are reset.

When the potential 3613 of the charge accumulation control signal line113 is set to a low level at time 3639 and then the potential 3614 ofthe reset signal line 114 is set to a low level at time 3640;accordingly, the reset operation is terminated, whereby the chargeaccumulation operation by the photodiode 101 starts.

When the potential 3613 of the charge accumulation control signal line113 is set to a high level again at the time 3633, the chargeaccumulation operation in the signal charge accumulation portion 112starts in all the pixels from the first row to the 480th row.

The following operation is the same as the operation mode in FIG. 10A.

In this manner, the pixel in the image sensor can be driven withoutusing a high-performance sequential circuit such as a shift register,reduction in power consumption and the area of the driver circuit, andimprovement in flexibility in layout of the circuit and the wiring canbe achieved.

As described above, when the transistor including an oxide semiconductoris used for the transistor used for forming the pixel in the imagesensor, the image sensor with global shutter can be easily realized,which can provide a semiconductor device capable of taking an imagewithout a distortion with respect to an object.

Note that the structure and operation of the image sensor in thisembodiment may be applied not only to an imaging device aimed only attaking an image but also to a touch panel and the like in which adisplay element of a display device is provided with an imaging element,for example.

This embodiment can be implemented in combination with any of the otherembodiments or the examples, as appropriate.

Embodiment 2

In this embodiment, a layout of a pixel circuit of a semiconductordevice in one embodiment of the present invention will be described.

As an example of the case where the pixel circuit of FIG. 8 is actuallyformed, the top view of the layout of a pixel circuit is illustrated inFIG. 11. Note that all the transistors used for the pixel circuit ofFIG. 8 are formed using a silicon semiconductor.

The pixel circuit illustrated in FIG. 11 is formed with a pin photodiode1801, an amplifying transistor 1802, a charge accumulation controltransistor 1803, a reset transistor 1804, a selection transistor 1805, acharge accumulation control signal line 1813, a reset signal line 1814,a selection signal line 1815, an output signal line 1820, a power supplyline 1830, and a ground potential line 1831. Layers illustrated in theview are an i-type silicon semiconductor layer 1241, a gate wiring layer1242, a wiring layer 1243, an n-type silicon semiconductor layer 1244,and a p-type silicon semiconductor layer 1245.

Of these, the i-type silicon semiconductor layer 1241, the n-typesilicon semiconductor layer 1244, and the p-type silicon semiconductorlayer 1245 are semiconductor layers forming the pin photodiode 1801. Asillustrated in a cross-sectional view of FIG. 12, a lateral junctionphotodiode is formed here. This lateral junction photodiode is anexample, and a stacked photodiode or a bulk buried photodiode can beemployed. Note that in the cross-sectional view of FIG. 12, a transistorincluding a silicon semiconductor is of SOI type; however, there is nolimitation thereto, and a bulk transistor may be used.

The gate wiring layer 1242 is connected to a gate electrode of theamplifying transistor 1802 and is connected to one of a source and adrain of the charge accumulation control transistor 1803 and one of asource and a drain of the reset transistor 1804 with the wiring layer1243. In addition, parts of these regions correspond to the signalcharge accumulation portion.

Next, as an example of the case where the pixel circuit of FIG. 9 isactually formed, the top view of the layout of a pixel circuit isillustrated in FIG. 13. Note that as for the transistors used for thepixel circuit of FIG. 9, the charge accumulation control transistor andthe reset transistor are formed using an oxide semiconductor, whereasthe amplifying transistor and the selection transistor are formed usinga silicon semiconductor.

The pixel circuit illustrated in FIG. 13 is formed with a pin photodiode1901, an amplifying transistor 1902, a charge accumulation controltransistor 1903, a reset transistor 1904, a selection transistor 1905, acharge accumulation control signal line 1913, a reset signal line 1914,a selection signal line 1915, an output signal line 1920, a power supplyline 1930, and a ground potential line 1931. Layers illustrated in theview are an i-type silicon semiconductor layer 1441, a gate wiring layer1442, a wiring layer 1443, an n-type silicon semiconductor layer 1444,and a p-type silicon semiconductor layer 1445.

Of these, the i-type silicon semiconductor layer 1441, the n-typesilicon semiconductor layer 1444, and the p-type silicon semiconductorlayer 1445 are semiconductor layers forming the pin photodiode 1901. Asillustrated in a cross-sectional view of FIG. 14, a lateral junctionphotodiode is formed here. This lateral junction photodiode is anexample, and a stacked photodiode or a bulk buried photodiode can beemployed. Note that in the cross-sectional view of FIG. 14, a transistorincluding a silicon semiconductor is of SOI type; however, there is nolimitation thereto, and a bulk transistor may be used.

The gate wiring layer 1442 is connected to a gate electrode of theamplifying transistor 1902 and is connected to one of a source and adrain of the charge accumulation control transistor 1903 and one of asource and a drain of the reset transistor 1904 with the wiring layer1443. In addition, parts of these regions correspond to the signalcharge accumulation portion.

As another example of the pixel structure, a pixel circuit illustratedin FIG. 15 can be given. The top view of the layout is illustrated inFIG. 16. Note that all the transistors used for the pixel circuit ofFIG. 15 are formed using an oxide semiconductor.

The pixel circuit illustrated in FIG. 16 is formed with a pin photodiode2801, an amplifying transistor 2802, a charge accumulation controltransistor 2803, a reset transistor 2804, a selection transistor 2805, acharge accumulation control signal line 2813, a reset signal line 2814,a selection signal line 2815, an output signal line 2820, a power supplyline 2830, and a ground potential line 2831. Layers illustrated in theview are an i-type silicon semiconductor layer 2941, a gate wiring layer2942, a wiring layer 2943, an n-type silicon semiconductor layer 2944,and a p-type silicon semiconductor layer 2945.

Of these, the i-type silicon semiconductor layer 2941, the n-typesilicon semiconductor layer 2944, and the p-type silicon semiconductorlayer 2945 are semiconductor layers forming the pin photodiode 2801. Asillustrated in a cross-sectional view of FIG. 17, a lateral junctionphotodiode is formed here. This lateral junction photodiode is anexample, and a stacked photodiode can also be employed.

The gate wiring layer 2942 is connected to a gate electrode of theamplifying transistor 2802 and is connected to one of a source and adrain of the charge accumulation control transistor 2803 and one of asource and a drain of the reset transistor 2804 with the wiring layer2943. In addition, parts of these regions correspond to the signalcharge accumulation portion.

There is the saturation electron number as one of important parameterswhich determine an imaging capability of a CCD sensor or a CMOS sensor.This saturation electron number corresponds to the amount of maximumcharge which can be held in the signal charge accumulation portion (FD)in the pixel in the CMOS sensor.

If charge lost from a capacitance (C) of the signal charge accumulationportion (FD) by off-state current (Ioff) of the transistor in a chargeholding period (Δt) is smaller than charge corresponding to voltage (ΔV)for one gray scale, the charge retention which does not have aninfluence on taking an image can be performed. A relational expressionof a capacitance value of the signal charge accumulation portion (FD)and off-state current (Ioff) at this time satisfies Ioff<C−ΔV/Δt.

In addition, in the case where a 10-bit gray scale is expressed, atleast 1023 electrons are needed. When the 10-bit gray scale is expressedusing 1023 electrons, an effect of an error increases, and the influenceof noise appears strongly. When the saturation electron number is verysmall, the influence of an optical shot noise is the strongest, wherebya statistical error is the square root of 1023. The electron number usedfor expressing one gray scale is increased approximately several timesas large as the minimum electron number, whereby the influence of theoptical shot noise can be reduced. Therefore, as the saturation electronnumber increases, the influence of the noise can be reduced.

Accordingly, in the case where each element is miniaturized to reducethe pixel area, a capacitance value also decreases; therefore, thesaturation electron number is reduced and there is a strong influence ofthe noise.

In one embodiment of the present invention, a transistor which is formedusing an oxide semiconductor and which has very low off-state current isused in a pixel; therefore, the saturation electron number for leakageis not needed to be considered. Accordingly, the pixel is easilyminiaturized. In addition, as compared to the case where a transistorwhich is formed using a silicon semiconductor is used in a pixel, thenoise resistance can be improved in a pixel with the same size.

This embodiment can be implemented in combination with any of the otherembodiments or the examples, as appropriate.

Embodiment 3

In this embodiment, a structure of a pixel circuit of a semiconductordevice which is one embodiment of the present invention will bedescribed.

In one embodiment of the present invention, various structures can beused for the pixel circuit of the semiconductor device. Although anexample based on the pixel circuit configuration illustrated in FIG. 1is used for description in Embodiments 1 and 2, another pixel circuitconfiguration is described in this embodiment.

Note that transistors and wirings in this embodiment are named forconvenience. Any of names is acceptable as long as functions of thetransistors and the wirings are described.

FIG. 18 is a pixel circuit configuration of four transistors, which issimilar to that in FIG. 1. A pixel circuit is formed with a photodiode1601, an amplifying transistor 1602, a charge accumulation controltransistor 1603, a reset transistor 1604, and a selection transistor1605. The circuit configuration of FIG. 18 is different from that ofFIG. 1 in the position of the selection transistor 1605.

A gate of the charge accumulation control transistor 1603 is connectedto a charge accumulation control signal line 1613, one of a source and adrain of the charge accumulation control transistor 1603 is connected toa cathode of the photodiode 1601, and the other of the source and thedrain of the charge accumulation control transistor 1603 is connected toa signal charge accumulation portion 1612. An anode of the photodiode1601 is connected to a ground potential line 1631.

A gate of the amplifying transistor 1602 is connected to the signalcharge accumulation portion 1612, one of a source and a drain of theamplifying transistor 1602 is connected to one of a source and a drainof the selection transistor 1605, and the other of the source and thedrain of the amplifying transistor 1602 is connected to an output signalline 1620.

A gate of the reset transistor 1604 is connected to a reset signal line1614, one of a source and a drain of the reset transistor 1604 isconnected to a power supply line 1630, and the other of the source andthe drain of the reset transistor 1604 is connected to the signal chargeaccumulation portion 1612.

A gate of the selection transistor 1605 is connected to a selectionsignal line 1615, and the other of the source and the drain of theselection transistor 1605 is connected to the power supply line 1630.Here, a charge holding capacitor may be connected between the signalcharge accumulation portion 1612 and the ground potential line 1631.

Next, functions of the elements forming the pixel circuit of FIG. 18 aredescribed. The photodiode 1601 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 1602outputs a signal which corresponds to a potential of the signal chargeaccumulation portion 1612. The charge accumulation control transistor1603 controls charge accumulation in the signal charge accumulationportion 1612 performed by the photodiode 1601. The reset transistor 1604controls the initialization of the potential of the signal chargeaccumulation portion 1612. The selection transistor 1605 controls theselection of the pixel in reading. The signal charge accumulationportion 1612 is a charge holding node and holds charge which variesdepending on the amount of light received by the photodiode 1601.

The charge accumulation control signal line 1613 is a signal line whichcontrols the charge accumulation control transistor 1603. The resetsignal line 1614 is a signal line which controls the reset transistor1604. The selection signal line 1615 is a signal line which controls theselection transistor 1605. The output signal line 1620 is a signal lineserving as an output destination of a signal generated by the amplifyingtransistor 1602. The power supply line 1630 is a signal line whichsupplies power supply voltage. The ground potential line 1631 is asignal line which sets a reference potential.

The operation of the pixel circuit illustrated in FIG. 18 is similar tothe operation of the pixel circuit illustrated in FIG. 1 described inEmbodiment 1.

Next, a pixel circuit configuration of three transistors illustrated inFIG. 19 is described. A pixel circuit is formed with a photodiode 1701,an amplifying transistor 1702, a charge accumulation control transistor1703, and a reset transistor 1704.

A gate of the charge accumulation control transistor 1703 is connectedto a charge accumulation control signal line 1713, one of a source and adrain of the charge accumulation control transistor 1703 is connected toa cathode of the photodiode 1701, and the other of the source and thedrain of the charge accumulation control transistor 1703 is connected toa signal charge accumulation portion 1712. An anode of the photodiode1701 is connected to a ground potential line 1731.

A gate of the amplifying transistor 1702 is connected to the signalcharge accumulation portion 1712, one of a source and a drain of theamplifying transistor 1702 is connected to a power supply line 1730, andthe other of the source and the drain of the amplifying transistor 1702is connected to an output signal line 1720.

A gate of the reset transistor 1704 is connected to a reset signal line1714, one of a source and a drain of the reset transistor 1704 isconnected to the power supply line 1730, and the other of the source andthe drain of the reset transistor 1704 is connected to the signal chargeaccumulation portion 1712. Here, a charge holding capacitor may beconnected between the signal charge accumulation portion 1712 and theground potential line 1731.

Next, functions of the elements forming the pixel circuit of FIG. 19 aredescribed. The photodiode 1701 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 1702outputs a signal which corresponds to a potential of the signal chargeaccumulation portion 1712. The charge accumulation control transistor1703 controls charge accumulation in the signal charge accumulationportion 1712 performed by the photodiode 1701. The reset transistor 1704controls the initialization of the potential of the signal chargeaccumulation portion 1712. The signal charge accumulation portion 1712is a charge holding node and holds charge which varies depending on theamount of light received by the photodiode 1701.

The charge accumulation control signal line 1713 is a signal line whichcontrols the charge accumulation control transistor 1703. The resetsignal line 1714 is a signal line which controls the reset transistor1704. The output signal line 1720 is a signal line serving as an outputdestination of a signal generated by the amplifying transistor 1702. Thepower supply line 1730 is a signal line which supplies power supplyvoltage. The ground potential line 1731 is a signal line which sets areference potential.

A pixel circuit configuration of three transistors, which is differentfrom that in FIG. 19, is illustrated in FIG. 20. A pixel circuit isformed with a photodiode 3801, an amplifying transistor 3802, a chargeaccumulation control transistor 3803, and a reset transistor 3804.

A gate of the charge accumulation control transistor 3803 is connectedto a charge accumulation control signal line 3813, one of a source and adrain of the charge accumulation control transistor 3803 is connected toa cathode of the photodiode 3801, and the other of the source and thedrain of the charge accumulation control transistor 3803 is connected toa signal charge accumulation portion 3812. An anode of the photodiode3801 is connected to a ground potential line 3831.

A gate of the amplifying transistor 3802 is connected to the signalcharge accumulation portion 3812, one of a source and a drain of theamplifying transistor 3802 is connected to a power supply line 3830, andthe other of the source and the drain of the amplifying transistor 3802is connected to an output signal line 3820.

A gate of the reset transistor 3804 is connected to a reset signal line3814, one of a source and a drain of the reset transistor 3804 isconnected to a reset power supply line 3832, and the other of the sourceand the drain of the reset transistor 3804 is connected to the signalcharge accumulation portion 3812. Here, a charge holding capacitor maybe connected between the signal charge accumulation portion 3812 and theground potential line 3831.

Next, functions of the elements forming the pixel circuit of FIG. 20 aredescribed. The photodiode 3801 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 3802outputs a signal which corresponds to a potential of the signal chargeaccumulation portion 3812. The charge accumulation control transistor3803 controls charge accumulation in the signal charge accumulationportion 3812 performed by the photodiode 3801. The reset transistor 3804controls the initialization of the potential of the signal chargeaccumulation portion 3812. The signal charge accumulation portion 3812is a charge holding node and holds charge which varies depending on theamount of light received by the photodiode 3801.

The charge accumulation control signal line 3813 is a signal line whichcontrols the charge accumulation control transistor 3803. The resetsignal line 3814 is a signal line which controls the reset transistor3804. The output signal line 3820 is a signal line serving as an outputdestination of a signal generated by the amplifying transistor 3802. Thereset power supply line 3832 is a power supply line which is differentfrom the power supply line 3830, and the reset power supply line 3832can initialize the potential of the signal charge accumulation portion3812, which is different from a potential of the power supply line 3830.The power supply line 3830 is a signal line which supplies power supplyvoltage. The ground potential line 3831 is a signal line which sets areference potential.

Next, the operations of the pixel circuits of FIG. 19 and FIG. 20 aredescribed using timing charts shown in FIGS. 21A and 21B. Note that theoperation of the circuit illustrated in FIG. 19 is basically the same asthat in FIG. 20; therefore, the structure of FIG. 19 is described here.

For simple description in FIGS. 21A and 21B, a potential 3913 of thecharge accumulation control signal line and a potential 3914 of thereset signal line are provided as signals which vary between two levels.Note that because each potential is an analog signal, the potential can,in practice, have various levels in accordance with situations withoutlimitation on two levels.

First, an operation mode according to FIG. 21A is described.

The potential 3913 of the charge accumulation control signal line 1713is set to a high level at time 3930. Next, when the potential 3914 ofthe reset signal line 1714 is set to a high level again at time 3931, apotential of the power supply line 1730 connected to one of the sourceand the drain of the reset transistor 1704 is supplied as a potential3912 of the signal charge accumulation portion 1712. These steps arereferred to as the reset operation.

When the potential 3914 of the reset signal line 1714 is set to a lowlevel at time 3932, the potential 3912 of the signal charge accumulationportion 1712 holds the same potential as the potential of the powersupply line 1730, whereby a reverse bias voltage is applied to thephotodiode 1701. At this stage, the accumulation operation starts.

Then, because the reverse current corresponding to the amount of lightflows to the photodiode 1701, the amount of charge accumulated in thesignal charge accumulation portion 1712 varies in accordance with theamount of light. At the same time, charge is supplied from the powersupply line 1730 to the output signal line 1720 in accordance with thepotential 3912 of the signal charge accumulation portion 1712. At thisstage, the read operation starts.

When the potential 3913 of the charge accumulation control signal line1713 is set to a low level at time 3933, transfer of charge from thesignal charge accumulation portion 1712 to the photodiode 1701 stops,whereby the amount of charge accumulated in the signal chargeaccumulation portion 1712 is determined. Here, the accumulationoperation is terminated.

Then, charge supplied from the power supply line 1730 to the outputsignal line 1720 is stopped, and a potential 3920 of the output signalline is determined. Here, the read operation is terminated.

Next, an operation mode according to FIG. 21B is described.

The potential 3913 of the charge accumulation control signal line 1713is set to a high level at the time 3930. Next, when the potential 3914of the reset signal line 1714 is set to a high level at the time 3931,the potential 3912 of the signal charge accumulation portion 1712 and apotential of the cathode of the photodiode 1701 are initialized to thepotential of the power supply line 1730 connected to one of the sourceand the drain of the reset transistor 1704. These steps are referred toas the reset operation.

When the potential 3913 of the charge accumulation control signal line1713 is set to a low level at time 3934 and then the potential 3914 ofthe reset signal line 1714 is set to a low level at time 3935, the resetoperation is terminated; accordingly, reverse current corresponding tothe amount of light flows to the photodiode 1701 to which the reversebias voltage is applied, whereby the potential of the cathode of thephotodiode 1701 varies.

When the potential 3913 of the charge accumulation control signal line1713 is set to a high level again at the time 3932, current flows by adifference in potential between the signal charge accumulation portion1712 and the cathode of the photodiode 1701, and the potential 3912 ofthe signal charge accumulation portion 1712 varies.

The steps after that are the same as those of the operation modeaccording to FIG. 21A.

Next, a pixel circuit configuration of three transistors, which isdifferent from that described above, is illustrated in FIG. 22. A pixelcircuit is formed with a photodiode 2001, an amplifying transistor 2002,a charge accumulation control transistor 2003, and a reset transistor2004. An anode of the photodiode 2001 is connected to a ground potentialline 2031.

A gate of the charge accumulation control transistor 2003 is connectedto a charge accumulation control signal line 2013, one of a source and adrain of the charge accumulation control transistor 2003 is connected toa cathode of the photodiode 2001, and the other of the source and thedrain of the charge accumulation control transistor 2003 is connected toa signal charge accumulation portion 2012.

A gate of the amplifying transistor 2002 is connected to the signalcharge accumulation portion 2012, one of a source and a drain of theamplifying transistor 2002 is connected to a power supply line 2030, andthe other of the source and the drain of the amplifying transistor 2002is connected to an output signal line 2020.

A gate of the reset transistor 2004 is connected to a reset signal line2014, one of a source and a drain of the reset transistor 2004 isconnected to the signal charge accumulation portion 2012, and the otherof the source and the drain of the reset transistor 2004 is connected tothe output signal line 2020. Here, a charge holding capacitor may beconnected between the signal charge accumulation portion 2012 and theground potential line 2031.

Next, functions of the elements forming the pixel circuit of FIG. 22 aredescribed. The photodiode 2001 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 2002outputs a signal which corresponds to a potential of the signal chargeaccumulation portion 2012. The charge accumulation control transistor2003 controls charge accumulation in the signal charge accumulationportion 2012 performed by the photodiode 2001. The reset transistor 2004controls the initialization of the potential of the signal chargeaccumulation portion 2012. The signal charge accumulation portion 2012is a charge holding node and holds charge which varies depending on theamount of light received by the photodiode 2001.

The charge accumulation control signal line 2013 is a signal line whichcontrols the charge accumulation control transistor 2003. The resetsignal line 2014 is a signal line which controls the reset transistor2004. The output signal line 2020 is a signal line serving as an outputdestination of a signal generated by the amplifying transistor 2002. Thepower supply line 2030 is a signal line which supplies power supplyvoltage. The ground potential line 2031 is a signal line which sets areference potential.

Next, the operation of the pixel circuit of FIG. 22 is described usingtiming charts shown in FIGS. 23A and 23B.

For simple description in FIGS. 23A and 23B, a potential 2113 of thecharge accumulation control signal line 2013 and a potential 2114 of thereset signal line 2014 are provided as signals which vary between twolevels. Note that because each potential is an analog signal, thepotential can, in practice, have various levels in accordance withsituations without limitation on two levels.

First, an operation mode according to FIG. 23A is described.

The potential 2113 of the charge accumulation control signal line 2013is set to a high level at time 2130. Next, when the potential 2114 ofthe reset signal line 2014 is set to a high level again at time 2131, areset potential is supplied from a potential 2120 of the output signalline 2020 connected to the other of the source and the drain of thereset transistor 2004 to the signal charge accumulation portion 2012 asa potential 2112 of the signal charge accumulation portion 2012. Thesesteps are referred to as the reset operation.

When the potential 2114 of the reset signal line 2014 is set to a lowlevel at time 2132, the potential 2112 of the signal charge accumulationportion 2012 holds the reset potential of the signal charge accumulationportion 2012, whereby a reverse bias voltage is applied to thephotodiode 2001. At this stage, the accumulation operation starts.

Then, because the reverse current corresponding to the amount of lightflows to the photodiode 2001, the amount of charge accumulated in thesignal charge accumulation portion 2012 varies in accordance with theamount of light. At the same time, charge is supplied from the powersupply line 2030 to the output signal line 2020 in accordance with thepotential 2112 of the signal charge accumulation portion 2012. At thisstage, the read operation starts.

When the potential 2113 of the charge accumulation control signal line2013 is set to a low level at time 2133, transfer of charge from thesignal charge accumulation portion 2012 to the photodiode 2001 stops,whereby the amount of charge accumulated in the signal chargeaccumulation portion 2012 is determined. Here, the accumulationoperation is terminated.

Then, charge supplied from the power supply line 2030 to the outputsignal line 2020 is stopped, and the potential 2120 of the output signalline 2020 is determined. Here, the read operation is terminated.

Next, an operation mode according to FIG. 23B is described.

The potential 2113 of the charge accumulation control signal line 2013is set to a high level at the time 2130. Next, when the potential 2114of the reset signal line 2014 is set to a high level at the time 2131,the potential 2112 of the signal charge accumulation portion 2012 and apotential of the cathode of the photodiode 2001 are initialized to thepotential 2120 of the output signal line 2020 connected to the other ofthe source and the drain of the reset transistor 2004. These steps arereferred to as the reset operation.

When the potential 2113 of the charge accumulation control signal line2013 is set to a low level at time 2134 and then the potential 2114 ofthe reset signal line 2014 is set to a low level at time 2135, the resetoperation is terminated; accordingly, reverse current corresponding tothe amount of light flows to the photodiode 2001 to which the reversebias voltage is applied, whereby the potential of the cathode of thephotodiode 2001 varies.

When the potential 2113 of the charge accumulation control signal line2013 is set to a high level again at the time 2132, current flows by adifference in potential between the signal charge accumulation portion2012 and the cathode of the photodiode 2001, and the potential 2112 ofthe signal charge accumulation portion 2012 varies.

The steps after that are the same as those of the operation modeaccording to FIG. 23A.

Next, a pixel circuit configuration of three transistors, which isdifferent from that described above, is illustrated in FIG. 24. A pixelcircuit is formed with a photodiode 2201, an amplifying transistor 2202,a charge accumulation control transistor 2203, and a selectiontransistor 2205. An anode of the photodiode 2201 is connected to a resetsignal line 2216.

A gate of the charge accumulation control transistor 2203 is connectedto a charge accumulation control signal line 2213, one of a source and adrain of the charge accumulation control transistor 2203 is connected toa cathode of the photodiode 2201, and the other of the source and thedrain of the charge accumulation control transistor 2203 is connected toa signal charge accumulation portion 2212.

A gate of the amplifying transistor 2202 is connected to the signalcharge accumulation portion 2212, one of a source and a drain of theamplifying transistor 2202 is connected to a power supply line 2230, andthe other of the source and the drain of the amplifying transistor 2202is connected to one of a source and a drain of the selection transistor2205.

A gate of the selection transistor 2205 is connected to a selectionsignal line 2215, and the other of the source and the drain of theselection transistor 2205 is connected to an output signal line 2220.Here, a charge holding capacitor may be connected between the signalcharge accumulation portion 2212 and a ground potential line.

Next, functions of the elements forming the pixel circuit of FIG. 24 aredescribed. The photodiode 2201 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 2202outputs a signal which corresponds to a potential of the signal chargeaccumulation portion 2212. The charge accumulation control transistor2203 controls charge accumulation in the signal charge accumulationportion 2212 performed by the photodiode 2201. The selection transistor2205 controls the selection of the pixel in reading. The signal chargeaccumulation portion 2212 is a charge holding node and holds chargewhich varies depending on the amount of light received by the photodiode2201.

The charge accumulation control signal line 2213 is a signal line whichcontrols the charge accumulation control transistor 2203. The resetsignal line 2216 is a signal line which supplies a reset potential tothe signal charge accumulation portion 2212. The output signal line 2220is a signal line serving as an output destination of a signal generatedby the amplifying transistor 2202. The selection signal line 2215 is asignal line which controls the selection transistor 2205. The powersupply line 2230 is a signal line which supplies power supply voltage.

Next, the operations of the pixel circuits of FIG. 24 are describedusing timing charts shown in FIGS. 25A and 25B.

For simple description in FIGS. 25A and 25B, a potential 2313 of thecharge accumulation control signal line 2213, a potential 2316 of thereset signal line 2216, and a potential 2315 of the selection signalline 2215 are provided as signals which vary between two levels. Notethat because each potential is an analog signal, the potential can, inpractice, have various levels in accordance with situations withoutlimitation on two levels.

First, an operation mode according to FIG. 25A is described.

The potential 2313 of the charge accumulation control signal line 2213is set to a high level at time 2330. Next, when the potential 2316 ofthe reset signal line 2216 is set to a high level at time 2331, apotential 2312 of the signal charge accumulation portion 2212 and apotential of the cathode of the photodiode 2201 are initialized to thepotential which is lower than the potential 2316 of the reset signalline 2216 by forward voltage of the photodiode 2201. These steps arereferred to as the reset operation.

When the potential 2316 of the reset signal line 2216 is set to a lowlevel at time 2332, the potential 2312 of the signal charge accumulationportion 2212 is kept at a high level, whereby a reverse bias voltage isapplied to the photodiode 2201. At this stage, the accumulationoperation starts.

Then, because the reverse current corresponding to the amount of lightflows to the photodiode 2201, the amount of charge accumulated in thesignal charge accumulation portion 2212 varies in accordance with theamount of light.

When the potential 2313 of the charge accumulation control signal line2213 is set to a low level at time 2333, transfer of charge from thesignal charge accumulation portion 2212 to the photodiode 2201 stops,whereby the amount of charge accumulated in the signal chargeaccumulation portion 2212 is determined. Here, the accumulationoperation is terminated.

When the potential 2315 of the selection signal line 2215 is set to ahigh level at time 2334, charge is supplied from the power supply line2230 to the output signal line 2220 in accordance with the potential2312 of the signal charge accumulation portion 2212. At this stage, theread operation starts.

When the potential 2315 of the selection signal line 2215 is set to alow level at time 2335, charge supplied from the power supply line 2230to the output signal line 2220 is stopped, and a potential 2320 of theoutput signal line 2220 is determined. Here, the read operation isterminated.

Next, an operation mode according to FIG. 25B is described.

The potential 2313 of the charge accumulation control signal line 2213is set to a high level at the time 2330. Next, when the potential 2316of the reset signal line 2216 is set to a high level at the time 2331,the potential 2312 of the signal charge accumulation portion 2212 andthe potential of the cathode of the photodiode 2201 are initialized tothe reset potential which is lower than the potential 2316 of the resetsignal line 2216 by forward voltage of the photodiode 2201. These stepsare referred to as the reset operation.

When the potential 2313 of the charge accumulation control signal line2213 is set to a low level at time 2336 and then the potential 2316 ofthe reset signal line 2216 is set to a low level at time 2337, the resetoperation is terminated; accordingly, reverse current corresponding tothe amount of light flows to the photodiode 2201 to which the reversebias voltage is applied, whereby the potential of the cathode of thephotodiode 2201 varies.

When the potential 2313 of the charge accumulation control signal line2213 is set to a high level again at the time 2332, current flows by adifference in potential between the signal charge accumulation portion2212 and the cathode of the photodiode 2201, and the potential 2312 ofthe signal charge accumulation portion 2212 varies.

The steps after that are the same as those of the operation modeaccording to FIG. 25A.

Next, a pixel circuit configuration of two transistors illustrated inFIG. 26 is described.

A pixel circuit is formed with a photodiode 4401, an amplifyingtransistor 4402, and a selection transistor 4405.

A gate of the amplifying transistor 4402 is connected to a signal chargeaccumulation control signal portion 4412, one of a source and a drain ofthe amplifying transistor 4402 is connected to a power supply line 4430,and the other of the source and the drain of the amplifying transistor4402 is connected to one of a source and a drain of the selectiontransistor 4405.

A gate of the selection transistor 4405 is connected to a selectionsignal line 4415, and the other of the source and the drain of theselection transistor 4405 is connected to an output signal line 4420.

A cathode of the photodiode 4401 is connected to the signal chargeaccumulation portion 4412, and an anode of the photodiode 4401 isconnected to a reset signal line 4416. Here, a charge holding capacitormay be connected between the signal charge accumulation portion 4412 anda ground potential line.

Next, a function of an element included in the pixel circuit in FIG. 26is described. The photodiode 4401 generates current in accordance withthe amount of light incident on the pixel. The amplifying transistor4402 outputs a signal which corresponds to the potential of a signalcharge accumulation portion 4412. The selection transistor 4405 controlsthe selection of the pixel in reading. The signal charge accumulationportion 4412 is a charge holding node and holds charge which variesdepending on the amount of light received by the photodiode 4401.

The reset signal line 4416 is a signal line which supplies a resetpotential to the signal charge accumulation portion 4412. The outputsignal line 4420 is a signal line to serve as an output destination of asignal generated by the amplifying transistor 4402. The selection signalline 4415 is a signal line which controls the selection transistor 4405.The power supply line 4430 is a signal line which supplies power supplyvoltage.

Next, the operations of the pixel circuits of FIG. 26 will be describedusing timing charts shown in FIG. 27.

For simple description in FIG. 27, a potential 3716 of the reset signalline 4416 and a potential 3715 of the selection signal line 4415 areprovided as signals which vary between two levels. Note that becauseeach potential is an analog signal, the potential can, in practice, havevarious levels in accordance with situations without limitation on twolevels.

When the potential 3716 of the reset signal line 4416 is set to a highlevel at time 3730, a potential 3712 of the signal charge accumulationportion 4412 is initialized to the reset potential which is lower thanthe potential 3716 of the reset signal line 4416 by forward voltage ofthe photodiode 4401. These steps are referred to as the reset operation.

When the potential 3716 of the reset signal line 4416 is set to a lowlevel at time 3731, the potential 3712 of the signal charge accumulationportion 4412 holds the reset potential, whereby a reverse bias voltageis applied to the photodiode 4401. At this stage, the accumulationoperation starts.

Then, because the reverse current corresponding to the amount of lightflows to the photodiode 4401, the amount of charge accumulated in thesignal charge accumulation portion 4412 varies in accordance with theamount of light.

When the potential 3715 of the selection signal line 4415 is set to ahigh level at time 3732, charge is supplied from the power supply line4430 to the output signal line 4420 in accordance with the potential3712 of the signal charge accumulation portion 4412. At this stage, theread operation starts.

When the potential 3715 of the selection signal line 4415 is set to alow level at time 3733, transfer of charge from the signal chargeaccumulation portion 4412 to the photodiode 4401 stops, whereby theamount of charge accumulated in the signal charge accumulation portion4412 is determined. Here, the accumulation operation is terminated.

Then, charge supply from the power supply line 4430 to the output signalline 4420 is stopped, and a potential 3720 of the output signal line isdetermined. Here, the read operation is terminated.

Next, a pixel circuit configuration of one transistor is illustrated inFIG. 28. The pixel circuit includes a photodiode 2601, an amplifyingtransistor 2602, and a capacitor 2606.

A gate of the amplifying transistor 2602 is connected to a signal chargeaccumulation portion 2612, one of a source and a drain of the amplifyingtransistor 2602 is connected to a power supply line 2630, and the otherof the source and the drain of the amplifying transistor 2602 isconnected to an output signal line 2620.

A cathode of the photodiode 2601 is connected to the signal chargeaccumulation portion 2612, and an anode of the photodiode 2601 isconnected to a reset signal line 2616. One of terminals of the capacitor2606 is connected to the signal charge accumulation portion 2612 and theother is connected to a selection signal line 2615. Here, a chargeholding capacitor is connected between the signal charge accumulationportion 2612 and a ground potential line.

Next, functions of the elements forming the pixel circuit of FIG. 28 aredescribed. The photodiode 2601 generates current in accordance with theamount of light incident on the pixel. The amplifying transistor 2602outputs a signal which corresponds to the potential of the signal chargeaccumulation portion 2612. The signal charge accumulation portion 2612is a charge holding node and holds charge which varies depending on theamount of light received by the photodiode 2601. Note that the selectionsignal line 2615 controls the potential of the signal chargeaccumulation portion 2612 with the use of capacitive coupling.

The reset signal line 2616 is a signal line which supplies a resetpotential to the signal charge accumulation portion 2612. The outputsignal line 2620 is a signal line serving as an output destination of asignal generated by the amplifying transistor 2602. The selection signalline 2615 is a signal line which controls the capacitor 2606. The powersupply line 2630 is a signal line which supplies power supply voltage.

Next, the operations of the pixel circuits of FIG. 28 will be describedusing timing charts shown in FIG. 29.

For simple description in FIG. 29, a potential 2716 of the reset signalline 2616 and a potential 2715 of the selection signal line 2615 areprovided as signals which vary between two levels. Note that becauseeach potential is an analog signal, the potential can, in practice, havevarious levels in accordance with situations without limitation on twolevels.

When the potential 2716 of the reset signal line 2616 is set to a highlevel at time 2730, a potential 2712 of the signal charge accumulationportion 2612 is initialized to the reset potential which is lower thanthe potential 2716 of the reset signal line 2616 by forward voltage ofthe photodiode 2601. These steps are referred to as the reset operation.

Next, when the potential 2716 of the reset signal line 2616 is set to alow level at time 2731, the potential 2712 of the signal chargeaccumulation portion 2612 holds the reset potential, whereby a reversebias voltage is applied to the photodiode 2601. At this stage, theaccumulation operation starts.

Then, because the reverse current corresponding to the amount of lightflows to the photodiode 2601, the amount of charge accumulated in thesignal charge accumulation portion 2612 varies in accordance with theamount of light.

The potential 2715 of the selection signal line 2615 is set to a highlevel at time 2732, so that the potential 2712 of the signal chargeaccumulation portion 2612 becomes higher due to capacitive coupling;accordingly, the amplifying transistor 2602 is turned on. Further,charge is supplied from the power supply line 2630 to the output signalline 2620 in accordance with the potential 2712 of the signal chargeaccumulation portion 2612. At this stage, the read operation starts.

When the potential 2715 of the selection signal line 2615 is set to alow level at time 2733, the potential 2712 of the signal chargeaccumulation portion 2612 is decreased by capacitive coupling andtransfer of charge from the signal charge accumulation portion 2612 tothe photodiode 2601 stops, whereby the amount of charge accumulated inthe signal charge accumulation portion 2612 is determined. Here, theaccumulation operation is terminated.

Then, charge supply from the power supply line 2630 to the output signalline 2620 is stopped, and a potential 2720 of the output signal line isdetermined. Here, the read operation is terminated.

Note that the pixel circuit structures in FIG. 26 and FIG. 28 havepreferably a structure of shielding incident light to the photodiodebecause charge of the signal charge accumulation portion flows outthrough the photodiode with the above structures.

This embodiment can be implemented in combination with any of the otherembodiments or the examples, as appropriate.

Embodiment 4

In this embodiment, conditions required by leakage current of atransistor used for a semiconductor device in one embodiment of thepresent invention will be described.

A transistor including a silicon semiconductor has high off-statecurrent. In the case where a CMOS sensor with global shutter which isformed using the transistor is operated, a charge holding period whichends when the last row is read gets longer and more charge flows due tothe off-state current in that period. The amount of charge is changed,which appears as a change in the gray scale of an image, and a normalimage is not obtained.

In this embodiment, in the case where a CMOS sensor with global shutteris used, conditions required for the off-state current of a transistorconnected to a signal charge accumulation portion in a pixel isdescribed.

Charge stored in the signal charge accumulation portion (FD) is lost bythe off-state current (Ioff) of the transistor connected to the signalcharge accumulation portion (FD). The amount of variations in chargewhich does not affect the gray scale of an image means the amount ofcharge (ΔQ_(FD)) in a charge holding period (Δt) is smaller than theamount of charge corresponding to voltage (ΔV_(FD)) which is changed byone gray scale of the capacitance (C_(FD)) of the signal chargeaccumulation portion (FD). The relation between the capacitance value(C_(FD)) of the signal charge accumulation portion (FD) and theoff-state current (Ioff) at this time is expressed by Formula 1.

C _(FD) ·ΔV _(FD) ≥I _(off) ·Δt=ΔQ _(FD)  Formula (1)

Here, when the maximum voltage (V_(FD)) of the signal chargeaccumulation portion (FD), a ratio (a) of effective value with respectto a change of one gray scale, and the number of n-bit gray scales(2^(n)) are used, the voltage (ΔV_(FD)) which is changed by one grayscale can be expressed by Formula 2.

ΔV _(FD) =V _(FD) ·a/2^(n)  Formula (2)

In addition, because the charge holding period (Δt) needs a chargeholding period for one frame at the maximum, the charge holding period(Δt) can be expressed by Formula 3 when a frame frequency (f) is used.

Δt=1/f  Formula (3)

Here, Formulae 1, 2, and 3 are arranged, and Formula 4 is obtained.

2^(n) ≤C _(FD) ·V _(FD) ·f·a/I _(off)  Formula (4)

FIG. 37 is a graph showing the case where the relational expression ofFormula 4 is expressed with an equal sign. The vertical axis representsthe number of gray scales (n) of an image, and the horizontal axisrepresents charge Q_(FD) (=C_(FD)·V_(FD)). Three curves show states inwhich the off-state currents (Ioff) of the transistor are different fromone another, and a curve 1101 shows 1 [fA], a curve 1102 shows 10 [fA],and a curve 1103 shows 100 [fA]. Areas below the curve 1101, the curve1102, and the curve 1103 respectively show the number of gray scaleswhich can be provided. Note that FIG. 37 shows calculation results ofthe case where the relations of f=60 [Hz] and a=50 [%] are satisfied.

It is found from FIG. 37 and Formula 4 that the number of gray scales(n) of an image is logarithmically proportional to the capacitance(C_(FD)) and the voltage (V_(FD)). Reduction in the pixel size isaccompanied with a decrease in the capacitance (C_(FD)). Reduction inpower consumption is accompanied with a decrease in voltage (V_(FD)).Therefore, it is necessary to decrease the off-state current (Ioff) inorder to realize improvement in quality of an image as well as reductionin the pixel size and power consumption. That is, when the off-statecurrent (Ioff) is reduced, the pixel size and power consumption can bedecreased; accordingly, an image sensor by which a high quality image istaken can be provided.

As an example, an image sensor with the condition in which the relationsof C_(FD)=20 [fF] and V_(FD)=3 [V] are satisfied. A point 1111 and apoint 1113 corresponding to this condition in FIG. 37 are described. Thenumber of gray scales n of an image at the point 1113 is 4.17 [bit],whereas the number thereof at the point 1111 is 10.81 [bit]. Therefore,it is necessary to employ a transistor whose Ioff is approximately lessthan or equal to 1 [fA] in order to provide an image sensor with globalshutter in which the relations of C_(FD)=20 [fF], V_(FD)=3 [V], and n=10[bit] are satisfied. Such a transistor with very low off-state currentcan be provided by use of a transistor including an oxide semiconductor.

The minimum value of the amount of charge corresponding to one grayscale is ideally the amount of charge for one electron (1 e=1.902×10⁻¹⁹[C]). Needless to say, because the noise caused by a statistical error,such as a variation in the electron number should be removed in anactual semiconductor device, several or more electrons are required inpractical. Here, when an ideal limit is considered, the charge (ΔQ_(FD))which is to be lost in the charge holding period should be smaller thanthe amount of charge for one electron (1 e). Accordingly, Formula 1 canbe expressed as Formula 5.

C _(FD) ·ΔV _(FD)=1e≥I _(off) ·Δt  Formula (5)

Further, Formula 5 can be expressed as Formula 6. FIG. 38 is a graphshowing the case where the relational expression of Formula 6 isexpressed with an equal sign. The vertical axis represents the off-statecurrent (Ioff) of the transistor, and the horizontal axis represents theframe frequency (f). For example, in the case where f is 60 [Hz]illustrated as a point 1201 in FIG. 38, the required off-state currentIoff of the transistor is less than or equal to 0.01 [fA](=1.902×10⁻¹⁹[C]×60 [Hz]).

I _(off)≤1e/Δt=1e·f  Formula (6)

That is, in order to realize a CMOS image sensor with global shutter, atransistor whose off-state current is less than or equal to 0.01 [fA]may be used as a transistor connected to the signal charge accumulationportion in the pixel. Such an image sensor becomes feasible by use of atransistor including an oxide semiconductor as a transistor whoseoff-state current is very low.

This embodiment can be implemented in combination with any of the otherembodiments or the examples, as appropriate.

Embodiment 5

In this embodiment, an example of a transistor including an oxidesemiconductor will be described.

There is no particular limitation on a structure of a transistorincluding an oxide semiconductor disclosed in this specification. Forexample, a staggered type transistor or a planar type transistor havinga top-gate structure or a bottom-gate structure can be employed.Further, the transistor may have a single gate structure including onechannel formation region, a double gate structure including two channelformation regions, or a triple gate structure including three channelformation regions.

FIGS. 30A to 30D each illustrate an example of a cross-sectionalstructure of a transistor.

Transistors illustrated in FIGS. 30A to 30D each include an oxidesemiconductor. An advantage of using an oxide semiconductor is thatrelatively high mobility and very low off-state current can be obtained;needless to say, other semiconductors can be used.

A transistor 3410 illustrated in FIG. 30A is one of bottom-gatetransistors, and is also referred to as an inverted staggeredtransistor.

The transistor 3410 includes, over a substrate 2400 having an insulatingsurface, a gate electrode layer 2401, a gate insulating layer 2402, anoxide semiconductor layer 2403, a source electrode layer 2405 a, and adrain electrode layer 2405 b. An insulating layer 2407 and a protectiveinsulating layer 2409 are formed so as to cover these.

A transistor 3420 illustrated in FIG. 30B is one of bottom-gatetransistors referred to as a channel-protective type and is alsoreferred to as an inverted staggered transistor.

The transistor 3420 includes, over the substrate 2400 having aninsulating surface, the gate electrode layer 2401, the gate insulatinglayer 2402, the oxide semiconductor layer 2403, an insulating layer 2427functioning as a channel protective layer which covers a channelformation region of the oxide semiconductor layer 2403, the sourceelectrode layer 2405 a, and the drain electrode layer 2405 b. Inaddition, the protective insulating layer 2409 is formed so as to coverthese.

A transistor 3430 illustrated in FIG. 30C is a bottom-gate transistor,and includes, over the substrate 2400 having an insulating surface, thegate electrode layer 2401, the gate insulating layer 2402, the sourceelectrode layer 2405 a, the drain electrode layer 2405 b, and the oxidesemiconductor layer 2403. In addition, the insulating layer 2407 and theprotective insulating layer 2409 are formed so as to cover these.

In the transistor 3430, the gate insulating layer 2402 is provided onand in contact with the substrate 2400 and the gate electrode layer2401, and the source electrode layer 2405 a and the drain electrodelayer 2405 b are provided on and in contact with the gate insulatinglayer 2402. Further, the oxide semiconductor layer 2403 is provided overthe gate insulating layer 2402, the source electrode layer 2405 a, andthe drain electrode layer 2405 b.

A transistor 3440 illustrated in FIG. 30D is a kind of top-gatetransistor. The transistor 3440 includes, over the substrate 2400 havingan insulating surface, an insulating layer 2437, the oxide semiconductorlayer 2403, the source electrode layer 2405 a, the drain electrode layer2405 b, the gate insulating layer 2402, and the gate electrode layer2401. A wiring layer 2436 a and a wiring layer 2436 b are provided to bein contact with and electrically connected to the source electrode layer2405 a and the drain electrode layer 2405 b, respectively.

In this embodiment, the oxide semiconductor layer 2403 is used as asemiconductor layer included in a transistor as described above. As anoxide semiconductor material used for the oxide semiconductor layer2403, any of the following metal oxides can be used: anIn—Sn—Ga—Zn—O-based metal oxide which is a four-component metal oxide;an In—Ga—Zn—O-based metal oxide, an In—Sn—Zn—O-based metal oxide, anIn—Al—Zn—O-based metal oxide, a Sn—Ga—Zn—O-based metal oxide, anAl—Ga—Zn—O-based metal oxide, and a Sn—Al—Zn—O-based metal oxide whichare three-component metal oxides; an In—Zn—O-based metal oxide, aSn—Zn—O-based metal oxide, an Al—Zn—O-based metal oxide, a Zn—Mg—O-basedmetal oxide, a Sn—Mg—O-based metal oxide, and an In—Mg—O-based metaloxide which are two-component metal oxides; an In—O-based metal oxide; aSn—O-based metal oxide; and a Zn—O-based metal oxide. Further, Si may becontained in the oxide semiconductor. Here, for example, anIn—Ga—Zn—O-based oxide semiconductor is an oxide containing at least In,Ga, and Zn, and there is no particular limitation on the compositionratio thereof. Further, the In—Ga—Zn—O-based oxide semiconductor maycontain an element other than In, Ga, and Zn.

For the oxide semiconductor layer 2403, a thin film represented by thechemical formula, InMO₃(ZnO)_(m) (m>0) can be used. Here, M representsone or more metal elements selected from Zn, Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In the transistors 3410, 3420, 3430, and 3440 each including the oxidesemiconductor layer 2403, the current value in an off state (off-statecurrent value) can be small. Accordingly, in the case where thetransistors 3410, 3420, 3430, and 3440 are connected to a charge storagenode, flow of charge can be prevented as much as possible.

In addition, each of the transistors 3410, 3420, 3430, and 3440 whichinclude the oxide semiconductor layer 2403 can operate at high speedbecause they can achieve field-effect mobility that is relativelyhigher. Therefore, a driver circuit portion which drives a pixel can beformed on one substrate of, for example, a display device, an imagingdevice, or the like; therefore, the number of components can be reduced.

As the substrate 2400 having an insulating surface, a glass substrateformed of barium borosilicate glass, aluminoborosilicate glass, or thelike can be used.

In the bottom-gate transistors 3410, 3420, and 3430, an insulating filmserving as a base film may be provided between the substrate and thegate electrode layer. The base film has a function of preventingdiffusion of an impurity element from the substrate, and can be formedto have a single-layer structure or a stacked structure using one ormore films selected from a silicon nitride film, a silicon oxide film, asilicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 2401 can be formed using a metal material suchas molybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium or an alloy material which contains any of thesematerials as its main component. The gate electrode layer 2401 is notlimited to a single layer, and a stacked layer of different films mayalso be used.

The gate insulating layer 2402 can be formed using a silicon oxidelayer, a silicon nitride layer, a silicon oxynitride layer, a siliconnitride oxide layer, an aluminum oxide layer, an aluminum nitride layer,an aluminum oxynitride layer, an aluminum nitride oxide layer, or ahafnium oxide layer, by a plasma-enhanced CVD method, a sputteringmethod, or the like. The gate insulating layer 2402 is not limited tothe single layer, and a stacked layer of different films may also beused. For example, by a plasma-enhanced CVD method, a silicon nitridelayer (SiN_(y) (y>0)) with a thickness greater than or equal to 50 nmand less than or equal to 200 nm is formed as a first gate insulatinglayer, and a silicon oxide layer (SiO_(x) (x>0)) with a thicknessgreater than or equal to 5 nm and less than or equal to 200 nm is formedas a second gate insulating layer over the first gate insulating layer,so that a gate insulating layer with a total thickness of 200 nm isformed.

As the conductive film used for the source electrode layer 2405 a andthe drain electrode layer 2405 b, for example, a film of an elementselected from Al, Cr, Cu, Ta, Ti, Mo, and W, a film of alloy containingany of these elements, or the like can be used. Alternatively, astructure may be employed in which a high-melting-point metal layer ofTi, Mo, W, or the like is stacked over and/or below a metal layer of Al,Cu, or the like. In addition, heat resistance can be improved by usingan Al material to which an element (Si, Nd, Sc, or the like) whichprevents generation of a hillock or a whisker in an Al film is added.

A material similar to that of the source electrode layer 2405 a and thedrain electrode layer 2405 b can be used for a conductive film such asthe wiring layer 2436 a and the wiring layer 2436 b which are connectedto the source electrode layer 2405 a and the drain electrode layer 2405b, respectively.

Alternatively, the conductive film to be the source electrode layer 2405a and the drain electrode layer 2405 b (including a wiring layer formedusing the same layer as the source and drain electrode layers) may beformed using a conductive metal oxide. As conductive metal oxide, indiumoxide (In₂O₃), tin oxide (SnO₂), zinc oxide (ZnO), indium oxide-tinoxide alloy (In₂O₃—SnO₂, which is abbreviated to ITO), indium oxide-zincoxide alloy (In₂O₃—ZnO), or any of these metal oxide materials in whichsilicon oxide is contained can be used.

As the insulating layers 2407, 2427, and 2437, an inorganic insulatingfilm typical examples of which are a silicon oxide film, a siliconoxynitride film, an aluminum oxide film, and an aluminum oxynitride filmcan be used.

As the protective insulating layer 2409, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

A planarization insulating film may be formed over the protectiveinsulating layer 2409 in order to reduce surface unevenness caused bythe structure of the transistor. As the planarization insulating film,an organic material such as polyimide, acrylic, or benzocyclobutene canbe used. Other than such organic materials, it is also possible to use alow-dielectric constant material (a low-k material) or the like. Notethat the planarization insulating film may be formed by stacking aplurality of insulating films formed from these materials.

Thus, a high-performance semiconductor device can be provided by using atransistor including an oxide semiconductor layer described in thisembodiment.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Embodiment 6

In this embodiment, an example of a method for manufacturing atransistor including an oxide semiconductor layer will be described indetail with reference to drawings.

FIGS. 31A to 31E are cross-sectional views illustrating an example of aprocess of manufacturing a transistor 2510. The transistor 2510 is aninverted staggered transistor having a bottom-gate structure, which issimilar to the transistor 3410 illustrated in FIG. 30A.

An oxide semiconductor used for a semiconductor layer in this embodimentis an i-type (intrinsic) oxide semiconductor or a substantially i-type(intrinsic) oxide semiconductor. The i-type (intrinsic) oxidesemiconductor or substantially i-type (intrinsic) oxide semiconductor isobtained in such a manner that hydrogen, which serves as a donor, isremoved from an oxide semiconductor as much as possible, and the oxidesemiconductor is highly purified so as to contain as few impurities thatare not main components of the oxide semiconductor as possible. In otherwords, a feature is that a purified i-type (intrinsic) semiconductor, ora semiconductor close thereto, is obtained not by adding impurities butby removing impurities such as hydrogen or water as much as possible.Accordingly, the oxide semiconductor layer included in the transistor2510 is an oxide semiconductor layer which is highly purified and madeto be electrically i-type (intrinsic).

In addition, a purified oxide semiconductor includes extremely fewcarriers (close to zero), and the carrier concentration thereof is lowerthan 1×10¹⁴/cm³, preferably lower than 1×10¹²/cm³, more preferably1×10¹¹/cm³.

Because the oxide semiconductor includes extremely few carriers, theoff-state current can be reduced in a transistor. The smaller the amountof off-state current is, the better.

Specifically, in the transistor including the oxide semiconductor layer,off-state current density per micrometer in a channel width at roomtemperature can be less than or equal to 10 aA/μm (1×10⁻¹⁷ A/μm),further less than or equal to 1 aA/μm (1×10⁻¹⁸ A/μm), still further lessthan or equal to 10 zA/μm (1×10⁻²⁰ A/μm).

In addition, in the transistor 2510 including the oxide semiconductorlayer, the temperature dependence of on-state current is hardlyobserved, and the variations in off-state current are extremely small.

A process of manufacturing the transistor 2510 over a substrate 2505 isdescribed below with reference to FIGS. 31A to 31E.

First, a conductive film is formed over the substrate 2505 having aninsulating surface, and then a gate electrode layer 2511 is formed in afirst photolithography step and an etching step. Note that a resist maskmay be formed by an inkjet method. Formation of the resist mask by aninkjet method needs no photomask; thus, manufacturing cost can bereduced.

As the substrate 2505 having an insulating surface, a substrate similarto the substrate 2400 described in Embodiment 5 can be used. In thisembodiment, a glass substrate is used as the substrate 2505.

An insulating film serving as a base film may be provided between thesubstrate 2505 and the gate electrode layer 2511. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 2505, and can be formed to have a single-layer structure or astacked structure using one or more of a silicon nitride film, a siliconoxide film, a silicon nitride oxide film, and a silicon oxynitride film.

The gate electrode layer 2511 can be formed using a metal material suchas molybdenum, titanium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, or an alloy material which contains any of thesematerials as its main component. The gate electrode layer 2511 is notlimited to the single layer, and a stacked layer of different films mayalso be used.

Next, a gate insulating layer 2507 is formed over the gate electrodelayer 2511. The gate insulating layer 2507 can be formed using a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, an aluminum oxide layer, an aluminumnitride layer, an aluminum oxynitride layer, an aluminum nitride oxidelayer, or a hafnium oxide layer by a plasma-enhanced CVD method, asputtering method, or the like. The gate insulating layer 2507 is notlimited to the single layer, and a stacked layer of different films mayalso be used.

For the oxide semiconductor in this embodiment, an oxide semiconductorwhich is made to be an i-type semiconductor or a substantially i-typesemiconductor by removing impurities is used. Such a highly-purifiedoxide semiconductor is highly sensitive to an interface state andinterface charge; thus, an interface between the oxide semiconductorlayer and the gate insulating layer is important. For that reason, thegate insulating layer that is to be in contact with a highly-purifiedoxide semiconductor needs to have high quality.

For example, high-density plasma-enhanced CVD using microwaves (e.g., afrequency of 2.45 GHz) is preferable because a dense high-qualityinsulating layer having high withstand voltage can be formed. Thehighly-purified oxide semiconductor and the high-quality gate insulatinglayer are in close contact with each other, whereby the interface statecan be reduced and favorable interface characteristics can be obtained.

Needless to say, a different deposition method such as a sputteringmethod or a plasma-enhanced CVD method can be employed as long as ahigh-quality insulating layer can be formed as a gate insulating layer.Further, an insulating layer whose film quality and characteristic ofthe interface between the insulating layer and an oxide semiconductorare improved by heat treatment which is performed after formation of theinsulating layer may be formed as a gate insulating layer. In any case,any insulating layer may be used as long as the insulating layer hascharacteristics of enabling reduction in interface state density of theinterface between the insulating layer and an oxide semiconductor andformation of a favorable interface as well as having favorable filmquality as a gate insulating layer. An example of using a sputteringmethod is described here.

In order that hydrogen, hydroxyl, and moisture might be contained in thegate insulating layer 2507 and an oxide semiconductor film 2530 aslittle as possible, it is preferable that the substrate 2505 over whichthe gate electrode layer 2511 is formed or the substrate 2505 over whichlayers up to and including the gate insulating layer 2507 are formed bepreheated in a preheating chamber of a sputtering apparatus aspretreatment for deposition of the oxide semiconductor film 2530 so thatimpurities such as hydrogen or moisture adsorbed to the substrate 2505are eliminated and removed. As an exhaustion unit provided in thepreheating chamber, a cryopump is preferable. Note that this preheatingtreatment can be omitted. This preheating treatment may be similarlyperformed on the substrate 2505 over which layers up to and including asource electrode layer 2515 a and a drain electrode layer 2515 b areformed before formation of an insulating layer 2516.

Next, the oxide semiconductor film 2530 having a thickness greater thanor equal to 2 nm and less than or equal to 200 nm, preferably greaterthan or equal to 5 nm and less than or equal to 30 nm is formed over thegate insulating layer 2507 (see FIG. 31A).

Note that before the oxide semiconductor film 2530 is formed by asputtering method, powder substances (also referred to as particles ordust) attached on a surface of the gate insulating layer 2507 arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which an RF power source is used for application of voltage toa substrate side in an argon atmosphere so that ionized argon collideswith the substrate to modify a surface. Note that instead of argon,nitrogen, helium, oxygen, or the like may be used.

As an oxide semiconductor used for the oxide semiconductor film 2530,the oxide semiconductor described in Embodiment 5, such as afour-component metal oxide, a three-component metal oxide, atwo-component metal oxide, an In—O-based metal oxide, a Sn—O-based metaloxide, or a Zn—O-based metal oxide can be used. Further, Si may becontained in the oxide semiconductor. In this embodiment, the oxidesemiconductor film 2530 is deposited by a sputtering method using anIn—Ga—Zn—O-based oxide semiconductor target. Alternatively, the oxidesemiconductor film 2530 can be formed by a sputtering method in a raregas (typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere containing a rare gas (typically, argon) and oxygen.

As a target for forming the oxide semiconductor film 2530 by asputtering method, for example, a metal oxide having the followingcomposition ratio is used: the composition ratio of In₂O₃:Ga₂O₃:ZnO is1:1:1 [molar ratio]. Alternatively, a metal oxide having the followingcomposition ratio may be used: the composition ratio of In₂O₃:Ga₂O₃:ZnOis 1:1:2 [molar ratio]. The filling factor of such a target is from 90%to 100%, inclusive, preferably 95% to 99.9%, inclusive. With the use ofa metal oxide target with high filling factor, the deposited oxidesemiconductor film has high density.

In the case where an In—Zn—O-based material is used as the oxidesemiconductor, a target used has a composition ratio of In:Zn=50:1 to1:2 in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio),preferably In:Zn=20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2in a molar ratio), further preferably In:Zn=15:1 to 1.5:1(In₂O₃:ZnO=15:2 to 3:4 in a molar ratio). For example, in a target usedfor forming an In—Zn—O-based oxide semiconductor which has an atomicratio of In:Zn:O═X:Y:Z, the relation of Z>1.5X+Y is satisfied.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, hydroxyl, or hydride are removed be used as thesputtering gas for the deposition of the oxide semiconductor film 2530.

The substrate is placed in a deposition chamber under reduced pressure,and the substrate temperature is set to a temperature higher than orequal to 100° C. and lower than or equal to 600° C., preferably higherthan or equal to 200° C. and lower than or equal to 400° C. By formingthe oxide semiconductor film in a state where the substrate is heated,the impurity concentration in the formed oxide semiconductor film can bereduced. Moreover, damage to the film due to sputtering is reduced. Theoxide semiconductor film 2530 is formed over the substrate 2505 in sucha manner that a sputtering gas from which hydrogen and moisture havebeen removed is introduced into the deposition chamber while moistureremaining therein is removed, and the above-described target is used. Inorder to remove the moisture remaining in the deposition chamber, anentrapment vacuum pump, for example, a cryopump, an ion pump, or atitanium sublimation pump is preferably used. As an exhaustion unit, aturbo molecular pump to which a cold trap is added may be used. In thedeposition chamber which is evacuated with the cryopump, a hydrogenatom, a compound containing a hydrogen atom, such as water (H₂O), (morepreferably, also a compound containing a carbon atom), and the like areremoved, whereby the impurity concentration in the oxide semiconductorfilm formed in the deposition chamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulse direct current power source is preferable because powdersubstances (also referred to as particles or dust) generated indeposition can be reduced and the film thickness can be uniform.

Then, the oxide semiconductor film 2530 is processed into anisland-shaped oxide semiconductor layer in a second photolithographystep and an etching step. Here, a resist mask used for formation of theisland-shaped oxide semiconductor layer may be formed by an inkjetmethod. Formation of the resist mask by an inkjet method needs nophotomask; thus, manufacturing cost can be reduced.

In the case where a contact hole is formed in the gate insulating layer2507, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film 2530.

Note that the etching of the oxide semiconductor film 2530 may be dryetching, wet etching, or both dry etching and wet etching. As an etchantused for wet etching of the oxide semiconductor film 2530, for example,a mixed solution of phosphoric acid, acetic acid, and nitric acid, orthe like can be used. Alternatively, ITO-07N (produced by KANTO CHEMICALCO., INC.) may be used.

Next, the oxide semiconductor layer is subjected to first heattreatment. The oxide semiconductor layer can be dehydrated ordehydrogenated by this first heat treatment. The first heat treatment isperformed at a temperature higher than or equal to 400° C. and lowerthan or equal to 750° C., alternatively, higher than or equal to 400° C.and lower than the strain point of the substrate in an atmosphere ofnitrogen or a rare gas such as helium, neon, or argon. Here, thesubstrate is introduced into an electric furnace which is one of heattreatment apparatuses, and heat treatment is performed on the oxidesemiconductor layer at 450° C. for one hour in a nitrogen atmosphere;thus, an oxide semiconductor layer 2531 which is subjected todehydration or dehydrogenation is formed (see FIG. 31B).

Note that a heat treatment apparatus is not limited to an electricalfurnace, and may include a device for heating an object to be processedby heat conduction or heat radiation from a heating element such as aresistance heating element. For example, an RTA (rapid thermal anneal)apparatus such as a GRTA (gas rapid thermal anneal) apparatus or an LRTA(lamp rapid thermal anneal) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be processed by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high pressuresodium lamp, or a high pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As thehigh-temperature gas, an inert gas which does not react with an objectto be processed by heat treatment, such as nitrogen or a rare gas likeargon, is used.

For example, as the first heat treatment, GRTA may be performed asfollows. The substrate is transferred and put in an inert gas heated ata high temperature of 650° C. to 700° C., inclusive, is heated forseveral minutes, and is transferred and taken out of the inert gasheated at the high temperature.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in an inert gas which isintroduced into a heat treatment apparatus. Alternatively, the purity ofthe inert gas is preferably 6N (99.9999%) or more, more preferably 7N(99.99999%) or more (that is, the impurity concentration is 1 ppm orless, preferably 0.1 ppm or less).

Further, after the oxide semiconductor layer is heated in the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or anultra-dry air (the dew point is lower than or equal to −40° C.,preferably lower than or equal to −60° C.) may be introduced into thesame furnace. The purity of an oxygen gas or an N₂O gas which isintroduced into the heat treatment apparatus is preferably 6N or more,more preferably 7N or more (that is, the impurity concentration in theoxygen gas or the N₂O gas is 1 ppm or less, preferably 0.1 ppm or less).It is preferable that water, hydrogen, and the like be not contained inthese gases in particular. By the action of the oxygen gas or the N₂Ogas, oxygen which is a main component of the oxide semiconductor andwhich has been eliminated at the same time as the step for removingimpurities by dehydration or dehydrogenation can be supplied. Throughthis step, the oxide semiconductor layer can be highly purified and madeto be an electrically i-type (intrinsic) oxide semiconductor.

The first heat treatment for the oxide semiconductor layer can beperformed on the oxide semiconductor film 2530 that has not beenprocessed into the island-shaped oxide semiconductor layer. In thatcase, the substrate is taken out from the heat apparatus after the firstheat treatment, and then a photolithography step is performed.

Note that the first heat treatment may be performed at any of thefollowing timings in addition to the above timing as long as afterdeposition of the oxide semiconductor layer: after a source electrodelayer and a drain electrode layer are formed over the oxidesemiconductor layer and after an insulating layer is formed over thesource electrode layer and the drain electrode layer.

Further, in the case where a contact hole is formed in the gateinsulating layer 2507, the formation of the contact hole may beperformed either before or after the first heat treatment is performedon the oxide semiconductor film 2530.

An oxide semiconductor layer formed in the following manner may also beused: an oxide semiconductor is deposited twice, and heat treatment isperformed thereon twice. Through such steps, a crystal region which isc-axis-aligned perpendicularly to a surface of the film and has a largethickness can be formed without depending on a base component.

For example, a first oxide semiconductor film with a thickness greaterthan or equal to 3 nm and less than or equal to 15 nm is deposited, andfirst heat treatment is performed in a nitrogen, an oxygen, a rare gas,or a dry air atmosphere at a temperature higher than or equal to 450° C.and lower than or equal to 850° C. or preferably higher than or equal to550° C. and lower than or equal to 750° C., so that a first oxidesemiconductor film having a crystal region in a region including asurface is formed. Then, a second oxide semiconductor film which has alarger thickness than the first oxide semiconductor film is formed, andsecond heat treatment is performed at a temperature higher than or equalto 450° C. and lower than or equal to 850° C., preferably higher than orequal to 600° C. and lower than or equal to 700° C.

Through such steps, in the entire second oxide semiconductor film,crystal growth can proceed from the lower part to the upper part usingthe first oxide semiconductor film as a seed crystal, whereby an oxidesemiconductor layer having a thick crystal region can be formed.

Next, a conductive film to be the source electrode layer and the drainelectrode layer (including a wiring formed from the same layer as thesource electrode layer and the drain electrode layer) is formed over thegate insulating layer 2507 and the oxide semiconductor layer 2531. Asthe conductive film serving as the source electrode layer and the drainelectrode layer, a material which is similar to the material used forthe source electrode layer 2405 a and the drain electrode layer 2405 bdescribed in Embodiment 5 can be used.

A resist mask is formed over the conductive film in a thirdphotolithography step and selective etching is performed, so that thesource electrode layer 2515 a and the drain electrode layer 2515 b areformed. Then, the resist mask is removed (see FIG. 31C).

Light exposure at the time of the formation of the resist mask in thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. The channel length L of thetransistor that is completed later is determined by a distance betweenbottom end portions of the source electrode layer and the drainelectrode layer, which are adjacent to each other over the oxidesemiconductor layer 2531. In the case where the channel length L is lessthan 25 nm, the light exposure at the time of the formation of theresist mask in the third photolithography step may be performed usingextreme ultraviolet light having an extremely short wavelength ofseveral nanometers to several tens of nanometers. In the light exposureby extreme ultraviolet light, the resolution is high and the focus depthis large. Therefore, the channel length L of the transistor formed latercan be 10 nm to 1000 nm, inclusive, operation speed of the circuit canbe increased, and power consumption can be reduced because an off-statecurrent value is extremely low.

In order to reduce the number of photomasks and steps in thephotolithography step, the etching step may be performed using a resistmask formed by a multi-tone mask. Because light which passes through themulti-tone mask has a plurality of intensity levels, a resist mask whichpartly has a different thickness can be formed. The shape of the resistmask can be changed by ashing; therefore, a resist mask with differentshapes can be formed without a photolithography process being performed.Thus, the number of light-exposure masks can be reduced and the numberof corresponding photolithography steps can also be reduced, wherebysimplification of a process can be realized.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor layer 2531 when theconductive film is etched. However, it is difficult to obtain etchingconditions in which only the conductive film is etched and the oxidesemiconductor layer 2531 is not etched at all. In some cases, only partof the oxide semiconductor layer 2531 is etched to be an oxidesemiconductor layer having a groove portion (a recessed portion) whenthe conductive film is etched.

In this embodiment, a Ti film is used as the conductive film and anIn—Ga—Zn—O-based oxide semiconductor is used as the oxide semiconductorlayer 2531; thus, an ammonia hydrogen peroxide solution (a mixedsolution of ammonia, water, and a hydrogen peroxide solution) may beused as an etchant.

Next, the insulating layer 2516 serving as a protective insulating filmis formed in contact with part of the oxide semiconductor layer. Beforethe formation of the insulating layer 2516, plasma treatment using a gassuch as N₂O, N₂, or Ar may be performed to remove water or the likeadsorbed on an exposed surface of the oxide semiconductor layer.

The insulating layer 2516 can be formed to a thickness of at least 1 nmby a method through which impurities such as water or hydrogen do notenter the insulating layer 2516, such as a sputtering method, asappropriate. When hydrogen is contained in the insulating layer 2516,hydrogen might enter the oxide semiconductor layer or oxygen might beextracted from the oxide semiconductor layer by hydrogen. In such acase, the resistance of the oxide semiconductor layer on the backchannelside might be decreased (the oxide semiconductor layer on thebackchannel side might have n-type conductivity) and a parasitic channelmight be formed. Therefore, it is important to form the insulating layer2516 by a method through which hydrogen and impurities containinghydrogen are not contained therein.

In this embodiment, a silicon oxide film is formed to a thickness of 200nm as the insulating layer 2516 by a sputtering method. The substratetemperature in film formation may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The silicon oxide film can be deposited by a sputteringmethod in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas and oxygen. As atarget, a silicon oxide or silicon can be used. For example, with theuse of silicon for the target, a silicon oxide film can be formed bysputtering under an atmosphere containing oxygen. For the insulatinglayer 2516 which is formed in contact with the oxide semiconductorlayer, an inorganic insulating film that hardly contains impurities suchas moisture, a hydrogen ion, and hydroxyl and that blocks entry of suchimpurities from the outside is preferably used. Typically, a siliconoxide film, a silicon oxynitride film, an aluminum oxide film, analuminum oxynitride film, or the like can be used.

In order to remove moisture remaining in the deposition chamber forforming the insulating layer 2516 at the same time as deposition of theoxide semiconductor film 2530, an entrapment vacuum pump (such as acryopump) is preferably used. When the insulating layer 2516 isdeposited in the deposition chamber evacuated using a cryopump, theimpurity concentration in the insulating layer 2516 can be reduced. Inaddition, as an evacuation unit for removing moisture remaining in thedeposition chamber of the insulating layer 2516, a turbo molecular pumpprovided with a cold trap may be used.

It is preferable that a high-purity gas from which impurities such ashydrogen, water, hydroxyl, or hydride are removed be used as thesputtering gas for the deposition of the insulating layer 2516.

Next, second heat treatment is performed in an inert gas atmosphere oran oxygen gas atmosphere (preferably at a temperature of 200° C. to 400°C. inclusive, for example, 250° C. to 350° C. inclusive). For example,the second heat treatment is performed in a nitrogen atmosphere at 250°C. for one hour. In the second heat treatment, part of the oxidesemiconductor layer (channel formation region) is heated in the statewhere it is in contact with the insulating layer 2516.

Through the above steps, oxygen which is one of main components of theoxide semiconductor and which is reduced together with impurities suchas hydrogen, moisture, hydroxyl, or hydride (also referred to as ahydrogen compound) through the first heat treatment performed on theoxide semiconductor film can be supplied. Thus, the oxide semiconductorlayer is highly purified and is made to be an electrically i-type(intrinsic) semiconductor.

Through the above steps, the transistor 2510 is formed (see FIG. 31D).

When a silicon oxide layer having a lot of defects is used as the oxideinsulating layer, impurities such as hydrogen, moisture, hydroxyl, orhydride contained in the oxide semiconductor layer can be diffused intothe silicon oxide layer through the heat treatment performed after thesilicon oxide layer is formed. That is, the impurities in the oxidesemiconductor layer can be further reduced.

A protective insulating layer 2506 may be further formed over theinsulating layer 2516. For example, a silicon nitride film is formed bya sputtering method. An inorganic insulating film which hardly containsimpurities such as moisture and can prevents entry of the impuritiesfrom the outside, such as a silicon nitride film or an aluminum nitridefilm, is preferably used as the protective insulating layer. In thisembodiment, the protective insulating layer 2506 is formed using asilicon nitride film (see FIG. 31E).

A silicon nitride film used for the protective insulating layer 2506 isformed in such a manner that the substrate 2505 over which layers up toand including the insulating layer 2516 are formed is heated to higherthan or equal to 100° C. and lower than or equal to 400° C., asputtering gas containing high-purity nitrogen from which hydrogen andmoisture are removed is introduced, and a target of silicon is used.Also in that case, the protective insulating layer 2506 is preferablyformed while moisture remaining in the treatment chamber is removed,similarly to the insulating layer 2516.

After the protective insulating layer is formed, heat treatment may befurther performed at a temperature higher than or equal to 100° C. andlower than or equal to 200° C. in air for longer than or equal to 1 hourand shorter than or equal to 30 hours. This heat treatment may beperformed at a fixed temperature. Alternatively, the following change intemperature is set as one cycle and may be repeated plural times: thetemperature is increased from room temperature to a heating temperatureand then decreased to room temperature.

In this manner, with the use of the transistor including ahighly-purified oxide semiconductor layer manufactured using thisembodiment, the value of current in an off state (an off-state currentvalue) can be further reduced.

In addition, because the transistor including a highly-purified oxidesemiconductor layer has high field-effect mobility, high-speed operationis possible. Therefore, a driver circuit portion can be formed on onesubstrate of, for example, a display device or the like; therefore, thenumber of components can be reduced.

This embodiment can be implemented in combination with any of the otherembodiments, as appropriate.

Example 1

In this example, charge holding characteristics of an image sensorhaving a pixel circuit configuration which is one embodiment of thepresent invention will be described.

FIGS. 32A and 32B are circuit diagrams based on FIG. 24 according toEmbodiment 3. In FIG. 32A, a transistor including an oxide semiconductoris used for a charge accumulation control transistor 6103, and atransistor including a silicon semiconductor is used for each of anamplifying transistor 6102 and a selection transistor 6105.

On the other hand, in FIG. 32B, a transistor including a siliconsemiconductor is used for all of a charge accumulation controltransistor 6203, an amplifying transistor 6202, and a selectiontransistor 6205.

In this example, image sensors are formed using the pixels illustratedin the circuit diagrams of FIGS. 32A and 32B, and results of comparisonbetween charge holding capabilities using output characteristics of theimage sensors are described.

The details of characteristics of the operations of the pixel circuitsin FIGS. 32A and 32B are described in Embodiment 3; therefore, thedescription is omitted here. Note that a potential in each signal lineis as follows.

First, as common potentials in the pixel circuits of FIGS. 32A and 32B,a power supply line was set to 1.8 V, a high level potential of a resetsignal line was set to 3.3 V, and a low level potential of the resetsignal line was set to 0 V.

Only a potential of a charge accumulation control signal line is not thesame in order to correspond to characteristics of each transistor. Inthe pixel circuit of FIG. 32A, a high level potential of the chargeaccumulation control signal line was set to 3.0 V, and a low levelpotential thereof was set to −1.5 V. In the pixel circuit of FIG. 32B, ahigh level potential of the charge accumulation control signal line wasset to 2.6 V, and a low level potential thereof was set to −0.8 V.

FIG. 33 is a timing chart which illustrates input signals of a chargeaccumulation control signal line (TX) and a reset signal line (RD).Here, a period in which the reset signal line has a high level potentialcorresponds to a reset period; a period during which the potential ofthe reset signal line is set at a low level and the potential of thecharge accumulation control signal line falls to a low level correspondsto a light-exposure period (accumulation period); and a period after thetime when the potential of the charge accumulation control signal lineis set to a low level corresponds to a holding period.

FIG. 34A shows output characteristics when the signal in FIG. 33 wasinput to an image sensor having the pixel circuit of FIG. 32A at eachilluminance. The illuminance used for a test is 0 lx, 160 lx, 470 lx,and 1000 lx. During the reset period, a similar output is shown in eachilluminance because the reset potential is supplied; and in thelight-exposure period, the output changes with different slopes areshown. Then, in the holding period, an output in each illuminance isheld. With such an operation, light intensity can be converted to asignal. Here, when the potential of the charge accumulation controlsignal line is set at a high level or a low level, a value of an outputvaries by an effect of capacitive coupling of a charge accumulationcontrol transistor; however, there is no influence on an output signalin the holding period.

FIG. 34B shows output characteristics at each illuminance describedabove, which are obtained for a long time. A dotted frame A of FIG. 34Acorresponds to a dotted frame A of FIG. 34B.

As is clear here, it is found that an output signal has almost no changewith respect to the temporal axis at any illuminance and an image sensorhaving the pixel circuit in FIG. 32A has extremely good holdingcharacteristics.

On the other hand, FIG. 35A shows output characteristics when the signalin FIG. 33 was input to an image sensor having the pixel circuit of FIG.32B at each illuminance. The illuminance used for a test is 0 lx, 160lx, 470 lx, and 1000 lx. It is found that different outputcharacteristics are shown at each illuminance as in FIG. 34A. Note that0 lx means a dark condition.

FIG. 35B shows output characteristics at each illuminance describedabove, which are obtained for a long time. Here, it is found that theoutput signal falls as time passes, which is largely different from thatof FIG. 34B. In particular, it is remarkable in the case of highilluminance. A signal corresponding to illuminance is held while havinga slope at the beginning of the holding time; however, the signal of1000 lx and the signal of 470 lx eventually overlap with each other.This means that both signals cannot be held and determination becomesimpossible.

In the case of 0 lx, the signal is held, which is caused by sufficientlysmall dark current of the photodiode. The cause of extremely weakholding capability of charge in the case of high illuminance is theleakage current of the transistor including a silicon semiconductor.Because the leakage current is high, when light current flows to thephotodiode, charge flows out by the leakage current of the transistor.Needless to say, charge similarly flows out in the case where thephotodiode has high dark current.

In this manner, because the transistor including an oxide semiconductorhas extremely low leakage current, a circuit with extremely high chargeholding capability as illustrated in FIG. 34B can be realized.Accordingly, it can be said that using a transistor including an oxidesemiconductor for a transistor connected to the signal chargeaccumulation portion of the pixel is useful for the global shuttersystem which needs a long charge holding period.

This example can be implemented in combination with any of theembodiments or the other example, as appropriate.

Example 2

A display device according to one embodiment of the present invention ischaracterized by obtaining image data with high resolution. Therefore,an electronic device using the display device according to oneembodiment of the present invention can be more sophisticated by addingthe display device as a component.

For example, the display device can be used for display devices, laptopcomputers, or image reproducing devices provided with recording media(typically, devices which reproduce the content of recording media suchas DVDs (digital versatile discs), and have displays for displaying thereproduced images). In addition to the above examples, as an electronicdevice which can include the display device according to one embodimentof the present invention, mobile phones, portable game machines,portable information terminals, e-book readers, video cameras, digitalstill cameras, goggle-type displays (head mounted displays), navigationsystems, audio reproducing devices (e.g., car audio components anddigital audio players), copiers, facsimiles, printers, multifunctionprinters, automated teller machines (ATM), vending machines, and thelike can be given. Specific examples of such an electronic device areillustrated in FIGS. 36A to 36D.

FIG. 36A illustrates a display device including a housing 5001, adisplay portion 5002, a supporting base 5003, and the like. The displaydevice according to one embodiment of the present invention can be usedfor the display portion 5002. The use of the display device according toone embodiment of the present invention for the display portion 5002 canprovide a display device capable of obtaining image data with highresolution and capable of being equipped with higher-functionalapplications. Note that the display device includes all display devicesfor displaying information, such as display devices for personalcomputers, display devices for receiving TV broadcasts, and displaydevices for displaying advertisements.

FIG. 36B illustrates a portable information terminal including a housing5101, a display portion 5102, a switch 5103, operation keys 5104, aninfrared rays port 5105, and the like. The display device according toone embodiment of the present invention can be used for the displayportion 5102. The use of the display device according to one embodimentof the present invention for the display portion 5102 can provide aportable information terminal capable of obtaining image data with highresolution and being equipped with higher-functional applications.

FIG. 36C illustrates an automated teller machine including a housing5201, a display portion 5202, a coin slot 5203, a bill slot 5204, a cardslot 5205, a bankbook slot 5206, and the like. The display deviceaccording to one embodiment of the present invention can be used for thedisplay portion 5202. The use of the display device according to oneembodiment of the present invention for the display portion 5202 canprovide an automated teller machine capable of obtaining image data withhigh resolution and being more sophisticated. The automated tellermachine using the display device according to one embodiment of thepresent invention can read information of living body such as a fingerprint, a face, a handprint, a palm print, a pattern of a hand vein, aniris, and the like which are used for biometrics with higher accuracy.Therefore, a false non-match rate which is caused by false recognitionof a person to be identified as a different person and a falseacceptance rate which is caused by false recognition of a differentperson as a person to be identified can be suppressed.

FIG. 36D illustrates a portable game machine including a housing 5301, ahousing 5302, a display portion 5303, a display portion 5304, amicrophone 5305, speakers 5306, an operation key 5307, a stylus 5308,and the like. The display device according to one embodiment of thepresent invention can be used for the display portion 5303 or thedisplay portion 5304. The use of the display device according to oneembodiment of the present invention for the display portion 5303 or thedisplay portion 5304 can provide a portable game machine capable ofobtaining image data with high resolution and being equipped withhigher-functional applications. Note that although the portable gamemachine illustrated in FIG. 36D includes the two display portions 5303and 5304, the number of display portions included in the portable gamemachine is not limited to two.

This example can be implemented in combination with any of the otherembodiments or the other example, as appropriate.

This application is based on Japanese Patent Application serial no.2010-050486 filed with the Japan Patent Office on Mar. 8, 2010, theentire contents of which are hereby incorporated by reference.

1. (canceled)
 2. A semiconductor device comprising: a photodiode; firstto fourth transistors; and a capacitor, wherein one of a source and adrain of the first transistor is electrically connected to thephotodiode, wherein the other of the source and the drain of the firsttransistor is electrically connected to a gate of the second transistor,wherein the other of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the thirdtransistor, wherein the other of the source and the drain of the firsttransistor is electrically connected to the capacitor, wherein one of asource and a drain of the second transistor is electrically connected toone of a source and a drain of the fourth transistor, and wherein achannel formation region in the third transistor comprises an oxidesemiconductor.
 3. The semiconductor device according to claim 2, whereinthe photodiode comprises a silicon.
 4. The semiconductor deviceaccording to claim 2, wherein a channel formation region in the firsttransistor comprises an oxide semiconductor.
 5. A semiconductor devicecomprising: a photodiode; first to fourth transistors; and a capacitor,wherein one of a source and a drain of the first transistor iselectrically connected to the photodiode, wherein the other of thesource and the drain of the first transistor is electrically connectedto a gate of the second transistor, wherein the other of the source andthe drain of the first transistor is electrically connected to one of asource and a drain of the third transistor, wherein the other of thesource and the drain of the first transistor is electrically connectedto the capacitor, wherein one of a source and a drain of the secondtransistor is electrically connected to one of a source and a drain ofthe fourth transistor, wherein a channel formation region in the thirdtransistor comprises an oxide semiconductor, and wherein off-statecurrent per micrometer in a channel width of the third transistor isless than or equal to 1×10⁻¹⁷ A/μm.
 6. The semiconductor deviceaccording to claim 5, wherein the photodiode comprises a silicon.
 7. Thesemiconductor device according to claim 5, wherein a channel formationregion in the first transistor comprises an oxide semiconductor.
 8. Asemiconductor device comprising: a photodiode; first to fourthtransistors; and a capacitor, wherein one of a source and a drain of thefirst transistor is electrically connected to the photodiode, whereinthe other of the source and the drain of the first transistor iselectrically connected to a gate of the second transistor, wherein theother of the source and the drain of the first transistor iselectrically connected to one of a source and a drain of the thirdtransistor, wherein the other of the source and the drain of the firsttransistor is electrically connected to the capacitor, wherein one of asource and a drain of the second transistor is electrically connected toone of a source and a drain of the fourth transistor, wherein a channelformation region in the third transistor comprises an oxidesemiconductor, and wherein a channel formation region in the fourthtransistor comprises silicon.
 9. The semiconductor device according toclaim 8, wherein the photodiode comprises a silicon.
 10. Thesemiconductor device according to claim 8, wherein a channel formationregion in the first transistor comprises an oxide semiconductor.